Patchwork [4/4] block: Optionally snapshot page contents to provide stable pages during write

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Submitter Darrick J. Wong
Date Dec. 13, 2012, 8:08 a.m.
Message ID <20121213080811.23360.98131.stgit@blackbox.djwong.org>
Download mbox | patch
Permalink /patch/205776/
State Superseded
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Comments

Darrick J. Wong - Dec. 13, 2012, 8:08 a.m.
Several complaints have been received regarding long file write latencies when
memory pages must be held stable during writeback.  Since it might not be
acceptable to stall programs for the entire duration of a page write (which may
take many milliseconds even on good hardware), enable a second strategy wherein
pages are snapshotted as part of submit_bio; the snapshot can be held stable
while writes continue.

This provides a band-aid to provide stable page writes on jbd without needing
to backport the fixed locking scheme in jbd2.  A mount option is added to ext4
to allow administrators to enable it there.

For those wondering about the ext3 bandage -- fixing the jbd locking (which was
done as part of ext4dev years ago) is a lot of surgery, and setting
PG_writeback on data pages when we actually hold the page lock dropped ext3
performance by nearly an order of magnitude.  If we're going to migrate iscsi
and raid to use stable page writes, the complaints about high latency will
likely return.  We might as well centralize their page snapshotting thing to
one place.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 arch/tile/Kconfig       |    6 ------
 block/blk-core.c        |    8 +++++---
 fs/ext3/super.c         |    1 +
 fs/ext4/super.c         |    7 ++++++-
 include/uapi/linux/fs.h |    1 +
 mm/Kconfig              |   10 ++++++++++
 mm/bounce.c             |   38 ++++++++++++++++++++++++++++++++++++--
 mm/page-writeback.c     |    4 ++++
 8 files changed, 63 insertions(+), 12 deletions(-)



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Andrew Lutomirski - Dec. 14, 2012, 1:48 a.m.
On 12/13/2012 12:08 AM, Darrick J. Wong wrote:
> Several complaints have been received regarding long file write latencies when
> memory pages must be held stable during writeback.  Since it might not be
> acceptable to stall programs for the entire duration of a page write (which may
> take many milliseconds even on good hardware), enable a second strategy wherein
> pages are snapshotted as part of submit_bio; the snapshot can be held stable
> while writes continue.
> 
> This provides a band-aid to provide stable page writes on jbd without needing
> to backport the fixed locking scheme in jbd2.  A mount option is added to ext4
> to allow administrators to enable it there.

I'm a bit confused as to what it has to do with ext3.  Wouldn't this be
useful as a mount option everywhere, though?

If this becomes widely used, would it be better to snapshot on
wait_for_stable_page instead of on io submission?

FWIW, I'm about to pound pretty hard on this whole patchset on a box
that doesn't need stable pages.  I'll let you know how it goes.

--Andy
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Darrick J. Wong - Dec. 14, 2012, 2:10 a.m.
On Thu, Dec 13, 2012 at 05:48:06PM -0800, Andy Lutomirski wrote:
> On 12/13/2012 12:08 AM, Darrick J. Wong wrote:
> > Several complaints have been received regarding long file write latencies when
> > memory pages must be held stable during writeback.  Since it might not be
> > acceptable to stall programs for the entire duration of a page write (which may
> > take many milliseconds even on good hardware), enable a second strategy wherein
> > pages are snapshotted as part of submit_bio; the snapshot can be held stable
> > while writes continue.
> > 
> > This provides a band-aid to provide stable page writes on jbd without needing
> > to backport the fixed locking scheme in jbd2.  A mount option is added to ext4
> > to allow administrators to enable it there.
> 
> I'm a bit confused as to what it has to do with ext3.  Wouldn't this be
> useful as a mount option everywhere, though?

ext3 requires snapshots; the rest are ok with either strategy.

*If* snapshotting is generally liked, then yes I'll go redo it as a vfs mount
option.

> If this becomes widely used, would it be better to snapshot on
> wait_for_stable_page instead of on io submission?

That really depends on how long you can afford to wait and how much free
memory you have. :)  It's all a big tradeoff between write latency and
consumption of memory pages and bandwidth, and one that I doubt I'm qualified
to make for everyone.

> FWIW, I'm about to pound pretty hard on this whole patchset on a box
> that doesn't need stable pages.  I'll let you know how it goes.

Yay!

--D
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Dave Chinner - Dec. 14, 2012, 3:33 a.m.
On Thu, Dec 13, 2012 at 06:10:49PM -0800, Darrick J. Wong wrote:
> On Thu, Dec 13, 2012 at 05:48:06PM -0800, Andy Lutomirski wrote:
> > On 12/13/2012 12:08 AM, Darrick J. Wong wrote:
> > > Several complaints have been received regarding long file write latencies when
> > > memory pages must be held stable during writeback.  Since it might not be
> > > acceptable to stall programs for the entire duration of a page write (which may
> > > take many milliseconds even on good hardware), enable a second strategy wherein
> > > pages are snapshotted as part of submit_bio; the snapshot can be held stable
> > > while writes continue.
> > > 
> > > This provides a band-aid to provide stable page writes on jbd without needing
> > > to backport the fixed locking scheme in jbd2.  A mount option is added to ext4
> > > to allow administrators to enable it there.
> > 
> > I'm a bit confused as to what it has to do with ext3.  Wouldn't this be
> > useful as a mount option everywhere, though?
> 
> ext3 requires snapshots; the rest are ok with either strategy.
> 
> *If* snapshotting is generally liked, then yes I'll go redo it as a vfs mount
> option.

It's copying every single IO, right? If so, then please don't
propagate any further than is necessary to fix the broken
filesystems...

Cheers,

Dave.
Darrick J. Wong - Dec. 14, 2012, 7:43 p.m.
On Fri, Dec 14, 2012 at 02:33:34PM +1100, Dave Chinner wrote:
> On Thu, Dec 13, 2012 at 06:10:49PM -0800, Darrick J. Wong wrote:
> > On Thu, Dec 13, 2012 at 05:48:06PM -0800, Andy Lutomirski wrote:
> > > On 12/13/2012 12:08 AM, Darrick J. Wong wrote:
> > > > Several complaints have been received regarding long file write latencies when
> > > > memory pages must be held stable during writeback.  Since it might not be
> > > > acceptable to stall programs for the entire duration of a page write (which may
> > > > take many milliseconds even on good hardware), enable a second strategy wherein
> > > > pages are snapshotted as part of submit_bio; the snapshot can be held stable
> > > > while writes continue.
> > > > 
> > > > This provides a band-aid to provide stable page writes on jbd without needing
> > > > to backport the fixed locking scheme in jbd2.  A mount option is added to ext4
> > > > to allow administrators to enable it there.
> > > 
> > > I'm a bit confused as to what it has to do with ext3.  Wouldn't this be
> > > useful as a mount option everywhere, though?
> > 
> > ext3 requires snapshots; the rest are ok with either strategy.
> > 
> > *If* snapshotting is generally liked, then yes I'll go redo it as a vfs mount
> > option.
> 
> It's copying every single IO, right? If so, then please don't
> propagate any further than is necessary to fix the broken
> filesystems...

Yup.  I wasn't intending this flag for general service, though I /am/ curious
to hear if anyone sees any substantive performance difference with snapshots.

--D
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com
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Andrew Lutomirski - Dec. 15, 2012, 1:12 a.m.
On Thu, Dec 13, 2012 at 6:10 PM, Darrick J. Wong
<darrick.wong@oracle.com> wrote:
> On Thu, Dec 13, 2012 at 05:48:06PM -0800, Andy Lutomirski wrote:
>> On 12/13/2012 12:08 AM, Darrick J. Wong wrote:
>> > Several complaints have been received regarding long file write latencies when
>> > memory pages must be held stable during writeback.  Since it might not be
>> > acceptable to stall programs for the entire duration of a page write (which may
>> > take many milliseconds even on good hardware), enable a second strategy wherein
>> > pages are snapshotted as part of submit_bio; the snapshot can be held stable
>> > while writes continue.
>> >
>> > This provides a band-aid to provide stable page writes on jbd without needing
>> > to backport the fixed locking scheme in jbd2.  A mount option is added to ext4
>> > to allow administrators to enable it there.
>>
>> I'm a bit confused as to what it has to do with ext3.  Wouldn't this be
>> useful as a mount option everywhere, though?
>
> ext3 requires snapshots; the rest are ok with either strategy.
>
> *If* snapshotting is generally liked, then yes I'll go redo it as a vfs mount
> option.
>
>> If this becomes widely used, would it be better to snapshot on
>> wait_for_stable_page instead of on io submission?
>
> That really depends on how long you can afford to wait and how much free
> memory you have. :)  It's all a big tradeoff between write latency and
> consumption of memory pages and bandwidth, and one that I doubt I'm qualified
> to make for everyone.
>
>> FWIW, I'm about to pound pretty hard on this whole patchset on a box
>> that doesn't need stable pages.  I'll let you know how it goes.
>
> Yay!
>
> --D

It survived.  I hit at least one mm bug, but I really don't think it's
a problem with your code.  (I have not tried this workload on Linux
3.7 at all before.  It normally runs on 3.5.)  The box in question is
ext4 on LVM on dm-crypt on (hardware) RAID 5 on hpsa, which should not
need stable pages.

The majority of the data written (that wasn't unlinked before it was
dropped from cache) was checksummed when written and verified later.
Most of this data was written using mmap.  This workload hammers the
vm concurrently in several threads, and it frequently stalls when
stable pages are enabled, so it's probably exercising the code
decently well.

Feel free to add Tested-by: Andy Lutomirski <luto@amacapital.net>

--Andy
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Darrick J. Wong - Dec. 15, 2012, 2:01 a.m.
On Fri, Dec 14, 2012 at 05:12:37PM -0800, Andy Lutomirski wrote:
> On Thu, Dec 13, 2012 at 6:10 PM, Darrick J. Wong
> <darrick.wong@oracle.com> wrote:
> > On Thu, Dec 13, 2012 at 05:48:06PM -0800, Andy Lutomirski wrote:
> >> On 12/13/2012 12:08 AM, Darrick J. Wong wrote:
> >> > Several complaints have been received regarding long file write latencies when
> >> > memory pages must be held stable during writeback.  Since it might not be
> >> > acceptable to stall programs for the entire duration of a page write (which may
> >> > take many milliseconds even on good hardware), enable a second strategy wherein
> >> > pages are snapshotted as part of submit_bio; the snapshot can be held stable
> >> > while writes continue.
> >> >
> >> > This provides a band-aid to provide stable page writes on jbd without needing
> >> > to backport the fixed locking scheme in jbd2.  A mount option is added to ext4
> >> > to allow administrators to enable it there.
> >>
> >> I'm a bit confused as to what it has to do with ext3.  Wouldn't this be
> >> useful as a mount option everywhere, though?
> >
> > ext3 requires snapshots; the rest are ok with either strategy.
> >
> > *If* snapshotting is generally liked, then yes I'll go redo it as a vfs mount
> > option.
> >
> >> If this becomes widely used, would it be better to snapshot on
> >> wait_for_stable_page instead of on io submission?
> >
> > That really depends on how long you can afford to wait and how much free
> > memory you have. :)  It's all a big tradeoff between write latency and
> > consumption of memory pages and bandwidth, and one that I doubt I'm qualified
> > to make for everyone.
> >
> >> FWIW, I'm about to pound pretty hard on this whole patchset on a box
> >> that doesn't need stable pages.  I'll let you know how it goes.
> >
> > Yay!
> >
> > --D
> 
> It survived.  I hit at least one mm bug, but I really don't think it's
> a problem with your code.  (I have not tried this workload on Linux
> 3.7 at all before.  It normally runs on 3.5.)  The box in question is

Would you mind sending along the bug report so I can make sure?

> ext4 on LVM on dm-crypt on (hardware) RAID 5 on hpsa, which should not
> need stable pages.
> 
> The majority of the data written (that wasn't unlinked before it was
> dropped from cache) was checksummed when written and verified later.
> Most of this data was written using mmap.  This workload hammers the
> vm concurrently in several threads, and it frequently stalls when
> stable pages are enabled, so it's probably exercising the code
> decently well.

Did you observe any change in performance?

> Feel free to add Tested-by: Andy Lutomirski <luto@amacapital.net>

Will do!  Thanks for the testing!

--D
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Andrew Lutomirski - Dec. 15, 2012, 2:06 a.m.
On Fri, Dec 14, 2012 at 6:01 PM, Darrick J. Wong
<darrick.wong@oracle.com> wrote:
> On Fri, Dec 14, 2012 at 05:12:37PM -0800, Andy Lutomirski wrote:
>> It survived.  I hit at least one mm bug, but I really don't think it's
>> a problem with your code.  (I have not tried this workload on Linux
>> 3.7 at all before.  It normally runs on 3.5.)  The box in question is
>
> Would you mind sending along the bug report so I can make sure?

http://marc.info/?l=linux-mm&m=135553342803210&w=2

>
>> ext4 on LVM on dm-crypt on (hardware) RAID 5 on hpsa, which should not
>> need stable pages.
>>
>> The majority of the data written (that wasn't unlinked before it was
>> dropped from cache) was checksummed when written and verified later.
>> Most of this data was written using mmap.  This workload hammers the
>> vm concurrently in several threads, and it frequently stalls when
>> stable pages are enabled, so it's probably exercising the code
>> decently well.
>
> Did you observe any change in performance?

No.  But I'm comparing to 3.5 + butchery to remove stable pages.  With
stable pages on, this workload performs terribly.  (It's a soft
real-time thing, as you can possibly guess from my domain name, and
various latency monitoring things go nuts when stable pages are
active.)

Actually, performance appears to be improved, probably due to
https://lkml.org/lkml/2012/12/14/14, which I tested concurrently.

>
>> Feel free to add Tested-by: Andy Lutomirski <luto@amacapital.net>
>
> Will do!  Thanks for the testing!

My pleasure.  When these changes go in to an upstream kernel, they'll
represent a significant reduction in how much our kernel differs from
kernel.org's :)  Thanks for fixing this.

--Andy
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Zheng Liu - Dec. 16, 2012, 4:13 p.m.
On Thu, Dec 13, 2012 at 12:08:11AM -0800, Darrick J. Wong wrote:
> diff --git a/mm/bounce.c b/mm/bounce.c
> index 0420867..fa11935 100644
> --- a/mm/bounce.c
> +++ b/mm/bounce.c
> @@ -178,6 +178,38 @@ static void bounce_end_io_read_isa(struct bio *bio, int err)
>  	__bounce_end_io_read(bio, isa_page_pool, err);
>  }
>  
> +#ifdef CONFIG_NEED_BOUNCE_POOL
> +static int might_snapshot_stable_page_write(struct bio **bio_orig)
> +{
> +	return bio_data_dir(*bio_orig) == WRITE;
> +}
> +
> +static int should_snapshot_stable_pages(struct page *page, int rw)
> +{
> +	struct backing_dev_info *bdi;
> +	struct address_space *mapping = page_mapping(page);
> +
> +	if (!mapping)
> +		return 0;
> +	bdi = mapping->backing_dev_info;
> +	if (!bdi_cap_stable_pages_required(bdi))
> +		return 0;
> +
> +	return mapping->host->i_sb->s_flags & MS_SNAP_STABLE &&
> +	       rw == WRITE;
> +}
> +#else
> +static int might_snapshot_stable_page_write(struct bio **bio_orig)
> +{
> +	return 0;
> +}
> +
> +static int should_snapshot_static_pages(struct page *page, int rw)
                              ^^^^^^
                              It should be _stable_.

Regards,
                                                - Zheng
> +{
> +	return 0;
> +}
> +#endif /* CONFIG_NEED_BOUNCE_POOL */
> +
>  static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
>  			       mempool_t *pool)
>  {
> @@ -192,7 +224,8 @@ static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
>  		/*
>  		 * is destination page below bounce pfn?
>  		 */
> -		if (page_to_pfn(page) <= queue_bounce_pfn(q))
> +		if (page_to_pfn(page) <= queue_bounce_pfn(q) &&
> +		    !should_snapshot_stable_pages(page, rw))
>  			continue;
>  
>  		/*
> @@ -284,7 +317,8 @@ void blk_queue_bounce(struct request_queue *q, struct bio **bio_orig)
>  	 * don't waste time iterating over bio segments
>  	 */
>  	if (!(q->bounce_gfp & GFP_DMA)) {
> -		if (queue_bounce_pfn(q) >= blk_max_pfn)
> +		if (queue_bounce_pfn(q) >= blk_max_pfn &&
> +		    !might_snapshot_stable_page_write(bio_orig))
>  			return;
>  		pool = page_pool;
>  	} else {
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Jan Kara - Dec. 17, 2012, 10:23 a.m.
On Thu 13-12-12 00:08:11, Darrick J. Wong wrote:
> Several complaints have been received regarding long file write latencies when
> memory pages must be held stable during writeback.  Since it might not be
> acceptable to stall programs for the entire duration of a page write (which may
> take many milliseconds even on good hardware), enable a second strategy wherein
> pages are snapshotted as part of submit_bio; the snapshot can be held stable
> while writes continue.
> 
> This provides a band-aid to provide stable page writes on jbd without needing
> to backport the fixed locking scheme in jbd2.  A mount option is added to ext4
> to allow administrators to enable it there.
> 
> For those wondering about the ext3 bandage -- fixing the jbd locking (which was
> done as part of ext4dev years ago) is a lot of surgery, and setting
> PG_writeback on data pages when we actually hold the page lock dropped ext3
> performance by nearly an order of magnitude.  If we're going to migrate iscsi
> and raid to use stable page writes, the complaints about high latency will
> likely return.  We might as well centralize their page snapshotting thing to
> one place.
  Umm, I kind of like this solution for ext3...

> diff --git a/mm/Kconfig b/mm/Kconfig
> index a3f8ddd..78db0e1 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -224,6 +224,16 @@ config BOUNCE
>  	def_bool y
>  	depends on BLOCK && MMU && (ZONE_DMA || HIGHMEM)
>  
> +# On the 'tile' arch, USB OHCI needs the bounce pool since tilegx will often
> +# have more than 4GB of memory, but we don't currently use the IOTLB to present
> +# a 32-bit address to OHCI.  So we need to use a bounce pool instead.
> +#
> +# We also use the bounce pool to provide stable page writes for users that
> +# don't (or can't) afford the wait latency.
> +config NEED_BOUNCE_POOL
> +	bool
> +	default y if (TILE && USB_OHCI_HCD) || (BLK_DEV_INTEGRITY && (EXT3_FS || EXT4_FS))
> +
  This means that NEED_BOUNCE_POOL is going to be enabled on pretty much
any distro kernel...

>  config NR_QUICK
>  	int
>  	depends on QUICKLIST
> diff --git a/mm/bounce.c b/mm/bounce.c
> index 0420867..fa11935 100644
> --- a/mm/bounce.c
> +++ b/mm/bounce.c
> @@ -178,6 +178,38 @@ static void bounce_end_io_read_isa(struct bio *bio, int err)
>  	__bounce_end_io_read(bio, isa_page_pool, err);
>  }
>  
> +#ifdef CONFIG_NEED_BOUNCE_POOL
> +static int might_snapshot_stable_page_write(struct bio **bio_orig)
> +{
> +	return bio_data_dir(*bio_orig) == WRITE;
> +}
... so might might_snapshot_stable_page_write() will be true for each
write. And thus blk_queue_bounce() becomes considerably more expensive?
Also calling should_snapshot_stable_pages() for every page seems to be
stupid since its result is going to be the same for all the pages in the
bio (well, I could imagine setups where it won't be but do we want to
support them?).

So cannot we just make a function like should_snapshot_stable_pages() to
test whether we really need the bouncing, use it in blk_queue_bounce() and
then pass the information to __blk_queue_bounce() if needed?

								Honza

> +static int should_snapshot_stable_pages(struct page *page, int rw)
> +{
> +	struct backing_dev_info *bdi;
> +	struct address_space *mapping = page_mapping(page);
> +
> +	if (!mapping)
> +		return 0;
> +	bdi = mapping->backing_dev_info;
> +	if (!bdi_cap_stable_pages_required(bdi))
> +		return 0;
> +
> +	return mapping->host->i_sb->s_flags & MS_SNAP_STABLE &&
> +	       rw == WRITE;
> +}
> +#else
> +static int might_snapshot_stable_page_write(struct bio **bio_orig)
> +{
> +	return 0;
> +}
> +
> +static int should_snapshot_static_pages(struct page *page, int rw)
> +{
> +	return 0;
> +}
> +#endif /* CONFIG_NEED_BOUNCE_POOL */
> +
>  static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
>  			       mempool_t *pool)
>  {
> @@ -192,7 +224,8 @@ static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
>  		/*
>  		 * is destination page below bounce pfn?
>  		 */
> -		if (page_to_pfn(page) <= queue_bounce_pfn(q))
> +		if (page_to_pfn(page) <= queue_bounce_pfn(q) &&
> +		    !should_snapshot_stable_pages(page, rw))
>  			continue;
>  
>  		/*
> @@ -284,7 +317,8 @@ void blk_queue_bounce(struct request_queue *q, struct bio **bio_orig)
>  	 * don't waste time iterating over bio segments
>  	 */
>  	if (!(q->bounce_gfp & GFP_DMA)) {
> -		if (queue_bounce_pfn(q) >= blk_max_pfn)
> +		if (queue_bounce_pfn(q) >= blk_max_pfn &&
> +		    !might_snapshot_stable_page_write(bio_orig))
>  			return;
>  		pool = page_pool;
>  	} else {
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 3e4a8cc..fbd8efb 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2291,6 +2291,10 @@ void wait_for_stable_page(struct page *page)
>  
>  	if (!bdi_cap_stable_pages_required(bdi))
>  		return;
> +#ifdef CONFIG_NEED_BOUNCE_POOL
> +	if (mapping->host->i_sb->s_flags & MS_SNAP_STABLE)
> +		return;
> +#endif /* CONFIG_NEED_BOUNCE_POOL */
>  
>  	wait_on_page_writeback(page);
>  }
>
Darrick J. Wong - Dec. 17, 2012, 10:54 p.m.
On Fri, Dec 14, 2012 at 06:06:50PM -0800, Andy Lutomirski wrote:
> On Fri, Dec 14, 2012 at 6:01 PM, Darrick J. Wong
> <darrick.wong@oracle.com> wrote:
> > On Fri, Dec 14, 2012 at 05:12:37PM -0800, Andy Lutomirski wrote:
> >> It survived.  I hit at least one mm bug, but I really don't think it's
> >> a problem with your code.  (I have not tried this workload on Linux
> >> 3.7 at all before.  It normally runs on 3.5.)  The box in question is
> >
> > Would you mind sending along the bug report so I can make sure?
> 
> http://marc.info/?l=linux-mm&m=135553342803210&w=2

Hm, this looks like a hugepages thing, which (afaik) doesn't touch fs code at
all.  Looks like this patchset is in the clear.

> >
> >> ext4 on LVM on dm-crypt on (hardware) RAID 5 on hpsa, which should not
> >> need stable pages.
> >>
> >> The majority of the data written (that wasn't unlinked before it was
> >> dropped from cache) was checksummed when written and verified later.
> >> Most of this data was written using mmap.  This workload hammers the
> >> vm concurrently in several threads, and it frequently stalls when
> >> stable pages are enabled, so it's probably exercising the code
> >> decently well.
> >
> > Did you observe any change in performance?
> 
> No.  But I'm comparing to 3.5 + butchery to remove stable pages.  With
> stable pages on, this workload performs terribly.  (It's a soft
> real-time thing, as you can possibly guess from my domain name, and
> various latency monitoring things go nuts when stable pages are
> active.)

Well, I guess that's good. :)

> Actually, performance appears to be improved, probably due to
> https://lkml.org/lkml/2012/12/14/14, which I tested concurrently.
> 
> >
> >> Feel free to add Tested-by: Andy Lutomirski <luto@amacapital.net>
> >
> > Will do!  Thanks for the testing!
> 
> My pleasure.  When these changes go in to an upstream kernel, they'll
> represent a significant reduction in how much our kernel differs from
> kernel.org's :)  Thanks for fixing this.

No problem!

--D
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Darrick J. Wong - Dec. 17, 2012, 10:56 p.m.
On Mon, Dec 17, 2012 at 12:13:36AM +0800, Zheng Liu wrote:
> On Thu, Dec 13, 2012 at 12:08:11AM -0800, Darrick J. Wong wrote:
> > diff --git a/mm/bounce.c b/mm/bounce.c
> > index 0420867..fa11935 100644
> > --- a/mm/bounce.c
> > +++ b/mm/bounce.c
> > @@ -178,6 +178,38 @@ static void bounce_end_io_read_isa(struct bio *bio, int err)
> >  	__bounce_end_io_read(bio, isa_page_pool, err);
> >  }
> >  
> > +#ifdef CONFIG_NEED_BOUNCE_POOL
> > +static int might_snapshot_stable_page_write(struct bio **bio_orig)
> > +{
> > +	return bio_data_dir(*bio_orig) == WRITE;
> > +}
> > +
> > +static int should_snapshot_stable_pages(struct page *page, int rw)
> > +{
> > +	struct backing_dev_info *bdi;
> > +	struct address_space *mapping = page_mapping(page);
> > +
> > +	if (!mapping)
> > +		return 0;
> > +	bdi = mapping->backing_dev_info;
> > +	if (!bdi_cap_stable_pages_required(bdi))
> > +		return 0;
> > +
> > +	return mapping->host->i_sb->s_flags & MS_SNAP_STABLE &&
> > +	       rw == WRITE;
> > +}
> > +#else
> > +static int might_snapshot_stable_page_write(struct bio **bio_orig)
> > +{
> > +	return 0;
> > +}
> > +
> > +static int should_snapshot_static_pages(struct page *page, int rw)
>                               ^^^^^^
>                               It should be _stable_.

Good catch!  Thank you!

--D
> 
> Regards,
>                                                 - Zheng
> > +{
> > +	return 0;
> > +}
> > +#endif /* CONFIG_NEED_BOUNCE_POOL */
> > +
> >  static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
> >  			       mempool_t *pool)
> >  {
> > @@ -192,7 +224,8 @@ static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
> >  		/*
> >  		 * is destination page below bounce pfn?
> >  		 */
> > -		if (page_to_pfn(page) <= queue_bounce_pfn(q))
> > +		if (page_to_pfn(page) <= queue_bounce_pfn(q) &&
> > +		    !should_snapshot_stable_pages(page, rw))
> >  			continue;
> >  
> >  		/*
> > @@ -284,7 +317,8 @@ void blk_queue_bounce(struct request_queue *q, struct bio **bio_orig)
> >  	 * don't waste time iterating over bio segments
> >  	 */
> >  	if (!(q->bounce_gfp & GFP_DMA)) {
> > -		if (queue_bounce_pfn(q) >= blk_max_pfn)
> > +		if (queue_bounce_pfn(q) >= blk_max_pfn &&
> > +		    !might_snapshot_stable_page_write(bio_orig))
> >  			return;
> >  		pool = page_pool;
> >  	} else {
> --
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Darrick J. Wong - Dec. 17, 2012, 11:20 p.m.
On Mon, Dec 17, 2012 at 11:23:59AM +0100, Jan Kara wrote:
> On Thu 13-12-12 00:08:11, Darrick J. Wong wrote:
> > Several complaints have been received regarding long file write latencies when
> > memory pages must be held stable during writeback.  Since it might not be
> > acceptable to stall programs for the entire duration of a page write (which may
> > take many milliseconds even on good hardware), enable a second strategy wherein
> > pages are snapshotted as part of submit_bio; the snapshot can be held stable
> > while writes continue.
> > 
> > This provides a band-aid to provide stable page writes on jbd without needing
> > to backport the fixed locking scheme in jbd2.  A mount option is added to ext4
> > to allow administrators to enable it there.
> > 
> > For those wondering about the ext3 bandage -- fixing the jbd locking (which was
> > done as part of ext4dev years ago) is a lot of surgery, and setting
> > PG_writeback on data pages when we actually hold the page lock dropped ext3
> > performance by nearly an order of magnitude.  If we're going to migrate iscsi
> > and raid to use stable page writes, the complaints about high latency will
> > likely return.  We might as well centralize their page snapshotting thing to
> > one place.
>   Umm, I kind of like this solution for ext3...

:)

> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index a3f8ddd..78db0e1 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -224,6 +224,16 @@ config BOUNCE
> >  	def_bool y
> >  	depends on BLOCK && MMU && (ZONE_DMA || HIGHMEM)
> >  
> > +# On the 'tile' arch, USB OHCI needs the bounce pool since tilegx will often
> > +# have more than 4GB of memory, but we don't currently use the IOTLB to present
> > +# a 32-bit address to OHCI.  So we need to use a bounce pool instead.
> > +#
> > +# We also use the bounce pool to provide stable page writes for users that
> > +# don't (or can't) afford the wait latency.
> > +config NEED_BOUNCE_POOL
> > +	bool
> > +	default y if (TILE && USB_OHCI_HCD) || (BLK_DEV_INTEGRITY && (EXT3_FS || EXT4_FS))
> > +
>   This means that NEED_BOUNCE_POOL is going to be enabled on pretty much
> any distro kernel...

I want to drop EXT4_FS from that default line.  Then we only need it in the
case where EXT4_USE_FOR_EXT23=y hasn't taken the place of EXT3_FS=y.

<shrug> Has anyone actually done that?

Heh, even *I* haven't done that.

> >  config NR_QUICK
> >  	int
> >  	depends on QUICKLIST
> > diff --git a/mm/bounce.c b/mm/bounce.c
> > index 0420867..fa11935 100644
> > --- a/mm/bounce.c
> > +++ b/mm/bounce.c
> > @@ -178,6 +178,38 @@ static void bounce_end_io_read_isa(struct bio *bio, int err)
> >  	__bounce_end_io_read(bio, isa_page_pool, err);
> >  }
> >  
> > +#ifdef CONFIG_NEED_BOUNCE_POOL
> > +static int might_snapshot_stable_page_write(struct bio **bio_orig)
> > +{
> > +	return bio_data_dir(*bio_orig) == WRITE;
> > +}
> ... so might might_snapshot_stable_page_write() will be true for each
> write. And thus blk_queue_bounce() becomes considerably more expensive?
> Also calling should_snapshot_stable_pages() for every page seems to be
> stupid since its result is going to be the same for all the pages in the
> bio (well, I could imagine setups where it won't be but do we want to
> support them?).
> 
> So cannot we just make a function like should_snapshot_stable_pages() to
> test whether we really need the bouncing, use it in blk_queue_bounce() and
> then pass the information to __blk_queue_bounce() if needed?

Yes.  I'd actually considered simply calling should_snapshot() on the first
bio_vec page and passing that information through, but thought I should play it
safe for the first revision.

However, a bio targets a single block device, so this seems like a safe
optimization.

--D
> 
> 								Honza
> 
> > +static int should_snapshot_stable_pages(struct page *page, int rw)
> > +{
> > +	struct backing_dev_info *bdi;
> > +	struct address_space *mapping = page_mapping(page);
> > +
> > +	if (!mapping)
> > +		return 0;
> > +	bdi = mapping->backing_dev_info;
> > +	if (!bdi_cap_stable_pages_required(bdi))
> > +		return 0;
> > +
> > +	return mapping->host->i_sb->s_flags & MS_SNAP_STABLE &&
> > +	       rw == WRITE;
> > +}
> > +#else
> > +static int might_snapshot_stable_page_write(struct bio **bio_orig)
> > +{
> > +	return 0;
> > +}
> > +
> > +static int should_snapshot_static_pages(struct page *page, int rw)
> > +{
> > +	return 0;
> > +}
> > +#endif /* CONFIG_NEED_BOUNCE_POOL */
> > +
> >  static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
> >  			       mempool_t *pool)
> >  {
> > @@ -192,7 +224,8 @@ static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
> >  		/*
> >  		 * is destination page below bounce pfn?
> >  		 */
> > -		if (page_to_pfn(page) <= queue_bounce_pfn(q))
> > +		if (page_to_pfn(page) <= queue_bounce_pfn(q) &&
> > +		    !should_snapshot_stable_pages(page, rw))
> >  			continue;
> >  
> >  		/*
> > @@ -284,7 +317,8 @@ void blk_queue_bounce(struct request_queue *q, struct bio **bio_orig)
> >  	 * don't waste time iterating over bio segments
> >  	 */
> >  	if (!(q->bounce_gfp & GFP_DMA)) {
> > -		if (queue_bounce_pfn(q) >= blk_max_pfn)
> > +		if (queue_bounce_pfn(q) >= blk_max_pfn &&
> > +		    !might_snapshot_stable_page_write(bio_orig))
> >  			return;
> >  		pool = page_pool;
> >  	} else {
> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > index 3e4a8cc..fbd8efb 100644
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> > @@ -2291,6 +2291,10 @@ void wait_for_stable_page(struct page *page)
> >  
> >  	if (!bdi_cap_stable_pages_required(bdi))
> >  		return;
> > +#ifdef CONFIG_NEED_BOUNCE_POOL
> > +	if (mapping->host->i_sb->s_flags & MS_SNAP_STABLE)
> > +		return;
> > +#endif /* CONFIG_NEED_BOUNCE_POOL */
> >  
> >  	wait_on_page_writeback(page);
> >  }
> > 
> -- 
> Jan Kara <jack@suse.cz>
> SUSE Labs, CR
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OGAWA Hirofumi - Dec. 27, 2012, 7:14 p.m.
"Darrick J. Wong" <darrick.wong@oracle.com> writes:

> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index 780d4c6..0144fbb 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -69,6 +69,7 @@ struct inodes_stat_t {
>  #define MS_REMOUNT	32	/* Alter flags of a mounted FS */
>  #define MS_MANDLOCK	64	/* Allow mandatory locks on an FS */
>  #define MS_DIRSYNC	128	/* Directory modifications are synchronous */
> +#define MS_SNAP_STABLE	256	/* Snapshot pages during writeback, if needed */
>  #define MS_NOATIME	1024	/* Do not update access times. */
>  #define MS_NODIRATIME	2048	/* Do not update directory access times */
>  #define MS_BIND		4096

I think this flag should be separated into "FS provide stable page" and
"FS needs bounce buffer for stable page".

My fs (I guess btrfs also) provides stable page by better way, and
doesn't need to wait writeback flags too. What needs is just to avoid
those stable page stuff.

Thanks.
Darrick J. Wong - Dec. 27, 2012, 9:40 p.m.
On Fri, Dec 28, 2012 at 04:14:49AM +0900, OGAWA Hirofumi wrote:
> "Darrick J. Wong" <darrick.wong@oracle.com> writes:
> 
> > diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> > index 780d4c6..0144fbb 100644
> > --- a/include/uapi/linux/fs.h
> > +++ b/include/uapi/linux/fs.h
> > @@ -69,6 +69,7 @@ struct inodes_stat_t {
> >  #define MS_REMOUNT	32	/* Alter flags of a mounted FS */
> >  #define MS_MANDLOCK	64	/* Allow mandatory locks on an FS */
> >  #define MS_DIRSYNC	128	/* Directory modifications are synchronous */
> > +#define MS_SNAP_STABLE	256	/* Snapshot pages during writeback, if needed */
> >  #define MS_NOATIME	1024	/* Do not update access times. */
> >  #define MS_NODIRATIME	2048	/* Do not update directory access times */
> >  #define MS_BIND		4096
> 
> I think this flag should be separated into "FS provide stable page" and
> "FS needs bounce buffer for stable page".
> 
> My fs (I guess btrfs also) provides stable page by better way, and
> doesn't need to wait writeback flags too. What needs is just to avoid
> those stable page stuff.

How does your fs (are we talking about vfat?) provide stable pages?

btrfs creates its own bdi and doesn't set the "stable pages required" flag, so
it already skips all the stable page stuff.

--D
> 
> Thanks.
> -- 
> OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
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OGAWA Hirofumi - Dec. 27, 2012, 9:48 p.m.
"Darrick J. Wong" <darrick.wong@oracle.com> writes:

>> I think this flag should be separated into "FS provide stable page" and
>> "FS needs bounce buffer for stable page".
>> 
>> My fs (I guess btrfs also) provides stable page by better way, and
>> doesn't need to wait writeback flags too. What needs is just to avoid
>> those stable page stuff.
>
> How does your fs (are we talking about vfat?) provide stable pages?

Basically it copies the data to another page before modifying data only
if page has writeback flag. (this is about new fs under development stage.)

> btrfs creates its own bdi and doesn't set the "stable pages required"
> flag, so it already skips all the stable page stuff.

I see.
Darrick J. Wong - Jan. 7, 2013, 8:44 p.m.
On Fri, Dec 28, 2012 at 06:48:35AM +0900, OGAWA Hirofumi wrote:
> "Darrick J. Wong" <darrick.wong@oracle.com> writes:
> 
> >> I think this flag should be separated into "FS provide stable page" and
> >> "FS needs bounce buffer for stable page".
> >> 
> >> My fs (I guess btrfs also) provides stable page by better way, and
> >> doesn't need to wait writeback flags too. What needs is just to avoid
> >> those stable page stuff.
> >
> > How does your fs (are we talking about vfat?) provide stable pages?
> 
> Basically it copies the data to another page before modifying data only
> if page has writeback flag. (this is about new fs under development stage.)

Aha, we're talking about tux3.  Since (afaik) there aren't any other
filesystems requesting this flag and tux3 uses the device bdi, should I just
create a patch and send it to you so it'll be included when tux3 goes in?

--D

> 
> > btrfs creates its own bdi and doesn't set the "stable pages required"
> > flag, so it already skips all the stable page stuff.
> 
> I see.
> -- 
> OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
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OGAWA Hirofumi - Jan. 8, 2013, 9:44 a.m.
"Darrick J. Wong" <darrick.wong@oracle.com> writes:

> On Fri, Dec 28, 2012 at 06:48:35AM +0900, OGAWA Hirofumi wrote:
>> "Darrick J. Wong" <darrick.wong@oracle.com> writes:
>> 
>> >> I think this flag should be separated into "FS provide stable page" and
>> >> "FS needs bounce buffer for stable page".
>> >> 
>> >> My fs (I guess btrfs also) provides stable page by better way, and
>> >> doesn't need to wait writeback flags too. What needs is just to avoid
>> >> those stable page stuff.
>> >
>> > How does your fs (are we talking about vfat?) provide stable pages?
>> 
>> Basically it copies the data to another page before modifying data only
>> if page has writeback flag. (this is about new fs under development stage.)
>
> Aha, we're talking about tux3.  Since (afaik) there aren't any other
> filesystems requesting this flag and tux3 uses the device bdi, should I just
> create a patch and send it to you so it'll be included when tux3 goes in?

If core doesn't provide, no need to waste your time. I will.

Thanks.

Patch

diff --git a/arch/tile/Kconfig b/arch/tile/Kconfig
index 875d008..c671fda 100644
--- a/arch/tile/Kconfig
+++ b/arch/tile/Kconfig
@@ -410,12 +410,6 @@  config TILE_USB
 	  Provides USB host adapter support for the built-in EHCI and OHCI
 	  interfaces on TILE-Gx chips.
 
-# USB OHCI needs the bounce pool since tilegx will often have more
-# than 4GB of memory, but we don't currently use the IOTLB to present
-# a 32-bit address to OHCI.  So we need to use a bounce pool instead.
-config NEED_BOUNCE_POOL
-	def_bool USB_OHCI_HCD
-
 source "drivers/pci/hotplug/Kconfig"
 
 endmenu
diff --git a/block/blk-core.c b/block/blk-core.c
index 3c95c4d..85e7705 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1433,6 +1433,11 @@  void blk_queue_bio(struct request_queue *q, struct bio *bio)
 	 */
 	blk_queue_bounce(q, &bio);
 
+	if (bio_integrity_enabled(bio) && bio_integrity_prep(bio)) {
+		bio_endio(bio, -EIO);
+		return;
+	}
+
 	if (bio->bi_rw & (REQ_FLUSH | REQ_FUA)) {
 		spin_lock_irq(q->queue_lock);
 		where = ELEVATOR_INSERT_FLUSH;
@@ -1673,9 +1678,6 @@  generic_make_request_checks(struct bio *bio)
 	 */
 	blk_partition_remap(bio);
 
-	if (bio_integrity_enabled(bio) && bio_integrity_prep(bio))
-		goto end_io;
-
 	if (bio_check_eod(bio, nr_sectors))
 		goto end_io;
 
diff --git a/fs/ext3/super.c b/fs/ext3/super.c
index 5366393..d251793 100644
--- a/fs/ext3/super.c
+++ b/fs/ext3/super.c
@@ -2068,6 +2068,7 @@  static int ext3_fill_super (struct super_block *sb, void *data, int silent)
 		test_opt(sb,DATA_FLAGS) == EXT3_MOUNT_JOURNAL_DATA ? "journal":
 		test_opt(sb,DATA_FLAGS) == EXT3_MOUNT_ORDERED_DATA ? "ordered":
 		"writeback");
+	sb->s_flags |= MS_SNAP_STABLE;
 
 	return 0;
 
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 80928f7..64d7bc8 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1222,7 +1222,7 @@  enum {
 	Opt_inode_readahead_blks, Opt_journal_ioprio,
 	Opt_dioread_nolock, Opt_dioread_lock,
 	Opt_discard, Opt_nodiscard, Opt_init_itable, Opt_noinit_itable,
-	Opt_max_dir_size_kb,
+	Opt_max_dir_size_kb, Opt_snap_stable,
 };
 
 static const match_table_t tokens = {
@@ -1297,6 +1297,7 @@  static const match_table_t tokens = {
 	{Opt_init_itable, "init_itable"},
 	{Opt_noinit_itable, "noinit_itable"},
 	{Opt_max_dir_size_kb, "max_dir_size_kb=%u"},
+	{Opt_snap_stable, "snapshot_stable"},
 	{Opt_removed, "check=none"},	/* mount option from ext2/3 */
 	{Opt_removed, "nocheck"},	/* mount option from ext2/3 */
 	{Opt_removed, "reservation"},	/* mount option from ext2/3 */
@@ -1478,6 +1479,7 @@  static const struct mount_opts {
 	{Opt_jqfmt_vfsv0, QFMT_VFS_V0, MOPT_QFMT},
 	{Opt_jqfmt_vfsv1, QFMT_VFS_V1, MOPT_QFMT},
 	{Opt_max_dir_size_kb, 0, MOPT_GTE0},
+	{Opt_snap_stable, 0, 0},
 	{Opt_err, 0, 0}
 };
 
@@ -1549,6 +1551,9 @@  static int handle_mount_opt(struct super_block *sb, char *opt, int token,
 			return -1;
 		*journal_ioprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, arg);
 		return 1;
+	case Opt_snap_stable:
+		sb->s_flags |= MS_SNAP_STABLE;
+		return 1;
 	}
 
 	for (m = ext4_mount_opts; m->token != Opt_err; m++) {
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 780d4c6..0144fbb 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -69,6 +69,7 @@  struct inodes_stat_t {
 #define MS_REMOUNT	32	/* Alter flags of a mounted FS */
 #define MS_MANDLOCK	64	/* Allow mandatory locks on an FS */
 #define MS_DIRSYNC	128	/* Directory modifications are synchronous */
+#define MS_SNAP_STABLE	256	/* Snapshot pages during writeback, if needed */
 #define MS_NOATIME	1024	/* Do not update access times. */
 #define MS_NODIRATIME	2048	/* Do not update directory access times */
 #define MS_BIND		4096
diff --git a/mm/Kconfig b/mm/Kconfig
index a3f8ddd..78db0e1 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -224,6 +224,16 @@  config BOUNCE
 	def_bool y
 	depends on BLOCK && MMU && (ZONE_DMA || HIGHMEM)
 
+# On the 'tile' arch, USB OHCI needs the bounce pool since tilegx will often
+# have more than 4GB of memory, but we don't currently use the IOTLB to present
+# a 32-bit address to OHCI.  So we need to use a bounce pool instead.
+#
+# We also use the bounce pool to provide stable page writes for users that
+# don't (or can't) afford the wait latency.
+config NEED_BOUNCE_POOL
+	bool
+	default y if (TILE && USB_OHCI_HCD) || (BLK_DEV_INTEGRITY && (EXT3_FS || EXT4_FS))
+
 config NR_QUICK
 	int
 	depends on QUICKLIST
diff --git a/mm/bounce.c b/mm/bounce.c
index 0420867..fa11935 100644
--- a/mm/bounce.c
+++ b/mm/bounce.c
@@ -178,6 +178,38 @@  static void bounce_end_io_read_isa(struct bio *bio, int err)
 	__bounce_end_io_read(bio, isa_page_pool, err);
 }
 
+#ifdef CONFIG_NEED_BOUNCE_POOL
+static int might_snapshot_stable_page_write(struct bio **bio_orig)
+{
+	return bio_data_dir(*bio_orig) == WRITE;
+}
+
+static int should_snapshot_stable_pages(struct page *page, int rw)
+{
+	struct backing_dev_info *bdi;
+	struct address_space *mapping = page_mapping(page);
+
+	if (!mapping)
+		return 0;
+	bdi = mapping->backing_dev_info;
+	if (!bdi_cap_stable_pages_required(bdi))
+		return 0;
+
+	return mapping->host->i_sb->s_flags & MS_SNAP_STABLE &&
+	       rw == WRITE;
+}
+#else
+static int might_snapshot_stable_page_write(struct bio **bio_orig)
+{
+	return 0;
+}
+
+static int should_snapshot_static_pages(struct page *page, int rw)
+{
+	return 0;
+}
+#endif /* CONFIG_NEED_BOUNCE_POOL */
+
 static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
 			       mempool_t *pool)
 {
@@ -192,7 +224,8 @@  static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
 		/*
 		 * is destination page below bounce pfn?
 		 */
-		if (page_to_pfn(page) <= queue_bounce_pfn(q))
+		if (page_to_pfn(page) <= queue_bounce_pfn(q) &&
+		    !should_snapshot_stable_pages(page, rw))
 			continue;
 
 		/*
@@ -284,7 +317,8 @@  void blk_queue_bounce(struct request_queue *q, struct bio **bio_orig)
 	 * don't waste time iterating over bio segments
 	 */
 	if (!(q->bounce_gfp & GFP_DMA)) {
-		if (queue_bounce_pfn(q) >= blk_max_pfn)
+		if (queue_bounce_pfn(q) >= blk_max_pfn &&
+		    !might_snapshot_stable_page_write(bio_orig))
 			return;
 		pool = page_pool;
 	} else {
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 3e4a8cc..fbd8efb 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2291,6 +2291,10 @@  void wait_for_stable_page(struct page *page)
 
 	if (!bdi_cap_stable_pages_required(bdi))
 		return;
+#ifdef CONFIG_NEED_BOUNCE_POOL
+	if (mapping->host->i_sb->s_flags & MS_SNAP_STABLE)
+		return;
+#endif /* CONFIG_NEED_BOUNCE_POOL */
 
 	wait_on_page_writeback(page);
 }