[RFC] Re: [BUG] ext4: cannot unfreeze a filesystem due to a deadlock

Hi,

On Tue, 19 Apr 2011 18:43:16 +0900
Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com> wrote:
> Hi,
> 
> (2011/04/18 19:51), Jan Kara wrote:
> > On Mon 18-04-11 18:05:01, Toshiyuki Okajima wrote:
> >>> On Fri 15-04-11 22:39:07, Toshiyuki Okajima wrote:
> >>>>>    For ext3 or ext4 without delayed allocation we block inside writepage()
> >>>>> function. But as I wrote to Dave Chinner, ->page_mkwrite() should probably
> >>>>> get modified to block while minor-faulting the page on frozen fs because
> >>>>> when blocks are already allocated we may skip starting a transaction and so
> >>>>> we could possibly modify the filesystem.
> >>>> OK. I think ->page_mkwrite() should also block writing the minor-faulting pages.
> >>>>
> >>>> (minor-pagefault)
> >>>> ->   do_wp_page()
> >>>>     ->   page_mkwrite(= ext4_mkwrite())
> >>>>        =>   BLOCK!
> >>>>
> >>>> (major-pagefault)
> >>>> ->   do_liner_fault()
> >>>>     ->   page_mkwrite(= ext4_mkwrite())
> >>>>        =>   BLOCK!
> >>>>
> >>>>>
> >>>>>>>> Mizuma-san's reproducer also writes the data which maps to the file (mmap).
> >>>>>>>> The original problem happens after the fsfreeze operation is done.
> >>>>>>>> I understand the normal write operation (not mmap) can be blocked while
> >>>>>>>> fsfreezing. So, I guess we don't always block all the write operation
> >>>>>>>> while fsfreezing.
> >>>>>>>    Technically speaking, we block all the transaction starts which means we
> >>>>>>> end up blocking all the writes from going to disk. But that does not mean
> >>>>>>> we block all the writes from going to in-memory cache - as you properly
> >>>>>>> note the mmap case is one of such exceptions.
> >>>>>> Hm, I also think we can allow the writes to in-memory cache but we can't allow
> >>>>>> the writes to disk while fsfreezing. I am considering that mmap path can
> >>>>>> write to disk while fsfreezing because this deadlock problem happens after
> >>>>>> fsfreeze operation is done...
> >>>>>    I'm sorry I don't understand now - are you speaking about the case above
> >>>>> when writepage() does not wait for filesystem being frozen or something
> >>>>> else?
> >>>> Sorry, I didn't understand around the page fault path.
> >>>> So, I had read the kernel source code around it, then I maybe understand...
> >>>>
> >>>> I worry whether we can update the file data in mmap case while fsfreezing.
> >>>> Of course, I understand that we can write to in-memory cache, and it is not a
> >>>> problem. However, if we can write to disk while fsfreezing, it is a problem.
> >>>> So, I summarize the cases whether we can write to disk or not.
> >>>>
> >>>> --------------------------------------------------------------------------
> >>>> Cases (Whether we can write the data mmapped to the file on the disk
> >>>> while fsfreezing)
> >>>>
> >>>> [1] One of the page which has been mmapped is not bound. And
> >>>>   the page is not allocated yet. (major fault?)
> >>>>
> >>>>     (1) user dirtys a page
> >>>>     (2) a page fault occurs (do_page_fault)
> >>>>     (3) __do_falut is called.
> >>>>     (4) ext4_page_mkwrite is called
> >>>>     (5) ext4_write_begin is called
> >>>>     (6) ext4_journal_start_sb       =>   We can STOP!
> >>>>
> >>>> [2] One of the page which has been mmapped is not bound. But
> >>>>   the page is already allocated, and the buffer_heads of the page
> >>>>   are not mapped (BH_Mapped).  (minor fault?)
> >>>>
> >>>>     (1) user dirtys a page
> >>>>     (2) a page fault occurs (do_page_fault)
> >>>>     (3) do_wp_page is called.
> >>>>     (4) ext4_page_mkwrite is called
> >>>>     (5) ext4_write_begin is called
> >>>>     (6) ext4_journal_start_sb       =>   We can STOP!
> >>>>
> >>>> [3] One of the page which has been mmapped is not bound. But
> >>>>   the page is already allocated, and the buffer_heads of the page
> >>>>   are mapped (BH_Mapped).  (minor fault?)
> >>>>
> >>>>     (1) user dirtys a page
> >>>>     (2) a page fault occurs (do_page_fault)
> >>>>     (3) do_wp_page is called.
> >>>>     (4) ext4_page_mkwrite is called
> >>>>     * Cannot block the dirty page to be written because all bh is mapped.
> >>>>     (5) user munmaps the page (munmap)
> >>>>     (6) zap_pte_range dirtys the page (struct page) which is pte_dirtyed.
> >>>>     (7) writeback thread writes the page (struct page) to disk
> >>>>                                             =>   We cannot STOP!
> >>>>
> >>>> [4] One of the page which has been mmapped is bound. And
> >>>>   the page is already allocated.
> >>>>
> >>>>     (1) user dirtys a page
> >>>>     ( ) no page fault occurs
> >>>>     (2) user munmaps the page (munmap)
> >>>>     (3) zap_pte_range dirtys the page (struct page) which is pte_dirtyed.
> >>>>     (4) writeback thread writes the page (struct page) to disk
> >>>>                                             =>   We cannot STOP!
> >>>> --------------------------------------------------------------------------
> >>>>
> >>>> So, we can block the cases [1], [2].
> >>>> But I think we cannot block the cases [3], [4] now.
> >>>> If fixing the page_mkwrite, we can also block the case [3].
> >>>> But the case [4] is not blocked because no page fault occurs
> >>>> when we dirty the mmapped page.
> >>>>
> >>>> Therefore, to repair this problem, we need to fix the cases [3], [4].
> >>>> I think we must modify the writeback thread to fix the case [4].
> >>>    The trick here is that when we write a page to disk, we write-protect
> >>> the page (you seem to call this that "the page is bound", I'm not sure why).
> >> Hm, I want to understand how to write-protect the page under fsfreezing.
> >    Look at what page_mkclean() called from clear_page_dirty_for_io() does...
> Thanks. I'll read that.
> 
> >
> >> But, anyway, I understand we don't need to consider the case [4].
> >    Yes.
> >
> >>> So we are guaranteed to receive a minor fault (case [3]) if user tries to
> >>> modify a page after we finish writeback while freezing the filesystem.
> >>> So principially all we need to do is just wait in ext4_page_mkwrite().
> >> OK. I understand.
> >> Are there any concrete ideas to fix this?
> >> For ext4, we can rescue from the case [3] by modifying ext4_page_mkwrite().
> >    Yes.
> >
> >> But for ext3 or other FSs, we must implement ->page_mkwrite() to prevent it?
> >    Sadly I don't see a simple way to fix this issue for all filesystems at
> > once. Implementing proper wait in block_page_mkwrite() should fix the issue
> > for xfs. Other filesystems like GFS2 or Btrfs will have to be fixed
> > separately as ext4. For ext3, we'd have to add ->page_mkwrite() support. I
> > have patches for this already for some time but I have to get to properly
> > testing them in more exotic conditions like 64k pages...
> OK. I understand the current status of your works to fix the problem which
> can be written with some data at mmap path while fsfreezing.
I have confirmed that the following patch works fine while my or
Mizuma-san's reproducer is running. Therefore,
 we can block to write the data, which is mmapped to a file, into a disk
by a page-fault while fsfreezing. 

I think this patch fixes the following two problems:
- A deadlock occurs between ext4_da_writepages() (called from
writeback_inodes_wb) and thaw_super(). (reported by Mizuma-san)
- We can also write the data, which is mmapped to a file,
  into a disk while fsfreezing (ext3/ext4).
                                       (reported by me)

Please examine this patch.

Thanks,
Toshiyuki Okajima
---
 fs/ext3/file.c          |   19 ++++++++++++-
 fs/ext3/inode.c         |   71 +++++++++++++++++++++++++++++++++++++++++++++++
 fs/ext4/inode.c         |    4 ++-
 include/linux/ext3_fs.h |    1 +
 4 files changed, 93 insertions(+), 2 deletions(-)

[RFC] Re: [BUG] ext4: cannot unfreeze a filesystem due to a deadlock

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