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

To: Mark Tinguely <tinguely@xxxxxxx>
Subject: Re: [PATCH] xfs: check all buffers in xfs_check_page_type()
From: Brian Foster <bfoster@xxxxxxxxxx>
Date: Fri, 28 Feb 2014 15:36:48 -0500
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <5310EB67.5050404@xxxxxxx>
References: <1393615369-41882-1-git-send-email-bfoster@xxxxxxxxxx> <5310EB67.5050404@xxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
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

> --Mark.
> 
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