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Re: [PATCH] xfs: check all buffers in xfs_check_page_type()

To: Brian Foster <bfoster@xxxxxxxxxx>
Subject: Re: [PATCH] xfs: check all buffers in xfs_check_page_type()
From: Mark Tinguely <tinguely@xxxxxxx>
Date: Fri, 28 Feb 2014 15:21:14 -0600
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140228203648.GC15562@xxxxxxxxxxxxxx>
References: <1393615369-41882-1-git-send-email-bfoster@xxxxxxxxxx> <5310EB67.5050404@xxxxxxx> <20140228203648.GC15562@xxxxxxxxxxxxxx>
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On 02/28/14 14:36, Brian Foster wrote:
On Fri, Feb 28, 2014 at 02:02:47PM -0600, Mark Tinguely wrote:
On 02/28/14 13:22, Brian Foster wrote:
xfs_aops_discard_page() was introduced in the following commit:

   xfs: truncate delalloc extents when IO fails in writeback

... to clean up left over delalloc ranges after I/O failure in
->writepage(). generic/224 tests for this scenario and occasionally
reproduces panics on sub-4k blocksize filesystems.

The cause of this is failure to clean up the delalloc range on a
page where the first buffer does not match one of the expected
states of xfs_check_page_type(). If a buffer is not unwritten,
delayed or dirty&mapped, xfs_check_page_type() stops and
immediately returns 0.

The stress test of generic/224 creates a scenario where the first
several buffers of a page with delayed buffers are mapped&uptodate
and some subsequent buffer is delayed. If the ->writepage() happens
to fail for this page, xfs_aops_discard_page() incorrectly skips
the entire page.

Modify xfs_aops_discard_page() to iterate all of the page buffers
to ensure a delayed buffer does not go undetected.

Signed-off-by: Brian Foster<bfoster@xxxxxxxxxx>
---

The only other caller to xfs_check_page_type() is xfs_convert_page(). I
think this is safe with respect to that codepath, given the additional
imap checks therein and whatnot, but thoughts appreciated.

Brian

  fs/xfs/xfs_aops.c | 2 --
  1 file changed, 2 deletions(-)

diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index db2cfb0..5962a9f 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -655,8 +655,6 @@ xfs_check_page_type(
                                acceptable += (type == XFS_IO_DELALLOC);
                        else if (buffer_dirty(bh)&&  buffer_mapped(bh))
                                acceptable += (type == XFS_IO_OVERWRITE);
-                       else
-                               break;
                } while ((bh = bh->b_this_page) != head);

                if (acceptable)

Is there any reason to scan all the buffers when we all we want is
an indication that at least one is acceptable? Maybe there are
generally not may buffers to a page to make it worthwhile.


Hi Mark,

Good point. We could pull the 'if (acceptable)' check up into the loop
and exit as soon as we find something writeable.

Alternatively, we could do something like the following and get rid of
'acceptable' entirely (or continue to nest the type checks if there's a
performance concern):

        ...
        if (buffer_unwritten(bh) && type == XFS_IO_UNWRITTEN)
                return 1;
        else if (buffer_delay(bh) && type == XFS_IO_DELALLOC)
                return 1;
        else if (buffer_dirty(bh) && buffer_mapped(bh)&&
                type == XFS_IO_OVERWRITE)
                return 1;
        ...

Brian

Good catch BTW. Yes that is what I was thinking.

--Mark.

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