ext4: fix race aio-dio vs freeze_fs

After freeze_fs was revoked (from Jan Kara) pages's write-back completion
is deffered before unwritten conversion, so explicit flush_unwritten_io()
was removed here: c724585b62411
But we still may face deferred conversion for aio-dio case
# Trivial testcase
for ((i=0;i<60;i++));do fsfreeze -f /mnt ;sleep 1;fsfreeze -u /mnt;done &
fio --bs=4k --ioengine=libaio --iodepth=128 --size=1g --direct=1 \
    --runtime=60 --filename=/mnt/file --name=rand-write --rw=randwrite
NOTE: Sane testcase should be integrated to xfstests, but it requires
changes in common/* code, so let's use this this test at the moment.

In order to fix this race we have to guard journal transaction with explicit
sb_{start,end}_intwrite()  as we do with ext4_evict_inode here:8e8ad8a5

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
---
 fs/ext4/extents.c |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)

Dmitry Monakhov <dmonakhov@openvz.org> writes:

> After freeze_fs was revoked (from Jan Kara) pages's write-back completion
> is deffered before unwritten conversion, so explicit flush_unwritten_io()
> was removed here: c724585b62411
> But we still may face deferred conversion for aio-dio case
> # Trivial testcase
> for ((i=0;i<60;i++));do fsfreeze -f /mnt ;sleep 1;fsfreeze -u /mnt;done &
> fio --bs=4k --ioengine=libaio --iodepth=128 --size=1g --direct=1 \
>     --runtime=60 --filename=/mnt/file --name=rand-write --rw=randwrite
> NOTE: Sane testcase should be integrated to xfstests, but it requires
> changes in common/* code, so let's use this this test at the moment.
>
> In order to fix this race we have to guard journal transaction with explicit
> sb_{start,end}_intwrite()  as we do with ext4_evict_inode here:8e8ad8a5
Fairly to say I'm not very happy with the fix because it continues bad
practice of ad-hock fixes for generic journal vs freeze synchronization

Ideal fix would be to move sb_start_intwrite/sb_end_intwrite() to
ext4_journal_start()/ext4_journal_stop() but this is not possible due to
limitations introduced by nojournal mode (described here:8e8ad8a5)
So let's fix nojournal instead. In order to do that we somehow have
store ref_count and pointer to sb inside nojournal_handle.
There are two possible ways to do that.
1) Embed second journal related field to task_struct and guard it with
   compile macros definition.
void *journal_info;
+ #ifdef CONFIG_EXTRA_JOURNAL_INFO
+   void *journal_info2;
+ #endif

2) Encode ref and sb in to single long. This can be done by aligning
   ext4_sb_info pointer to 4096. So we can embed ref count to lower bits
   like follows.
#define EXT4_NOJOURNAL_SHIFT 12
#define EXT4_NOJOURNAL_MAX_REF_COUNT 1 << (EXT4_NOJOURNAL_SHIFT-1)
#define EXT4_NOJOURNAL_MASK  (1 << EXT4_NOJOURNAL_SHIFT) -1
#define NOJOURNAL_SB(handle) (handle & ~EXT4_NOJOURNAL_MASK)
#define NOJOURNAL_REF(handle) ((handle & ~EXT4_NOJOURNAL_MASK) >> 1)
static int ext4_handle_valid(handle_t *handle)
{
        return !(handle & 0x1);
}
static handle_t *get_nojournal_handle(struct super_block *sb)
{
        handle_t *handle = current->journal_info;
        struct super_block *old_sb  = NOJOURNAL_SB(handle);
        unsigned long ref_cnt = NOJOURNAL_REF(handle);
        BUG_ON(old_sb && old_sb != sb);
        ref++;
        current->journal_info = NOJOURNAL_SB(handle);
}

What do you think about this? Are where any better way to fix this?

>
> Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
> ---
>  fs/ext4/extents.c |    7 +++++++
>  1 files changed, 7 insertions(+), 0 deletions(-)
>
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 3a6197a..4cba944 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -5040,6 +5040,12 @@ int ext4_convert_unwritten_extents(handle_t *handle, struct inode *inode,
>  	max_blocks = ((EXT4_BLOCK_ALIGN(len + offset, blkbits) >> blkbits) -
>  		      map.m_lblk);
>  	/*
> +	 * Protect us against freezing - AIO-DIO case. Caller didn't have to
> +	 * have any protection against it
> +	 */
> +	sb_start_intwrite(inode->i_sb);
> +
> +	/*
>  	 * This is somewhat ugly but the idea is clear: When transaction is
>  	 * reserved, everything goes into it. Otherwise we rather start several
>  	 * smaller transactions for conversion of each extent separately.
> @@ -5083,6 +5089,7 @@ int ext4_convert_unwritten_extents(handle_t *handle, struct inode *inode,
>  	}
>  	if (!credits)
>  		ret2 = ext4_journal_stop(handle);
> +	sb_end_intwrite(inode->i_sb);
>  	return ret > 0 ? ret2 : ret;
>  }
>  
> -- 
> 1.7.1
>
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On Mon 23-11-15 19:37:56, Dmitry Monakhov wrote:
> Dmitry Monakhov <dmonakhov@openvz.org> writes:
> 
> > After freeze_fs was revoked (from Jan Kara) pages's write-back completion
> > is deffered before unwritten conversion, so explicit flush_unwritten_io()
> > was removed here: c724585b62411
> > But we still may face deferred conversion for aio-dio case
> > # Trivial testcase
> > for ((i=0;i<60;i++));do fsfreeze -f /mnt ;sleep 1;fsfreeze -u /mnt;done &
> > fio --bs=4k --ioengine=libaio --iodepth=128 --size=1g --direct=1 \
> >     --runtime=60 --filename=/mnt/file --name=rand-write --rw=randwrite
> > NOTE: Sane testcase should be integrated to xfstests, but it requires
> > changes in common/* code, so let's use this this test at the moment.
> >
> > In order to fix this race we have to guard journal transaction with explicit
> > sb_{start,end}_intwrite()  as we do with ext4_evict_inode here:8e8ad8a5
> Fairly to say I'm not very happy with the fix because it continues bad
> practice of ad-hock fixes for generic journal vs freeze synchronization
> 
> Ideal fix would be to move sb_start_intwrite/sb_end_intwrite() to
> ext4_journal_start()/ext4_journal_stop() but this is not possible due to
> limitations introduced by nojournal mode (described here:8e8ad8a5)
> So let's fix nojournal instead. In order to do that we somehow have
> store ref_count and pointer to sb inside nojournal_handle.
> There are two possible ways to do that.
> 1) Embed second journal related field to task_struct and guard it with
>    compile macros definition.
> void *journal_info;
> + #ifdef CONFIG_EXTRA_JOURNAL_INFO
> +   void *journal_info2;
> + #endif
> 
> 2) Encode ref and sb in to single long. This can be done by aligning
>    ext4_sb_info pointer to 4096. So we can embed ref count to lower bits
>    like follows.
> #define EXT4_NOJOURNAL_SHIFT 12
> #define EXT4_NOJOURNAL_MAX_REF_COUNT 1 << (EXT4_NOJOURNAL_SHIFT-1)
> #define EXT4_NOJOURNAL_MASK  (1 << EXT4_NOJOURNAL_SHIFT) -1
> #define NOJOURNAL_SB(handle) (handle & ~EXT4_NOJOURNAL_MASK)
> #define NOJOURNAL_REF(handle) ((handle & ~EXT4_NOJOURNAL_MASK) >> 1)
> static int ext4_handle_valid(handle_t *handle)
> {
>         return !(handle & 0x1);
> }
> static handle_t *get_nojournal_handle(struct super_block *sb)
> {
>         handle_t *handle = current->journal_info;
>         struct super_block *old_sb  = NOJOURNAL_SB(handle);
>         unsigned long ref_cnt = NOJOURNAL_REF(handle);
>         BUG_ON(old_sb && old_sb != sb);
>         ref++;
>         current->journal_info = NOJOURNAL_SB(handle);
> }
> 
> What do you think about this? Are where any better way to fix this?

Well, we do have:

WARN_ON(sb->s_writers.frozen == SB_FREEZE_COMPLETE);

in ext4_journal_check_start() so we should get a warning for each
unexpected transaction started on frozen filesystem. The idea really is
there should be no write activity happening on frozen fs and the few cases
like MMP are handled by explicit sb_start_intwrite() calls. I'd prefer to
keep those cases special and don't add general freeze protection in
transaction handling code since that may only result in deadlocks instead
of the warning when starting transaction in unexpectected places...

								Honza