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Re: [PATCH, RFC] xfs: fix failed write handling

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH, RFC] xfs: fix failed write handling
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 9 Nov 2010 16:51:42 +1100
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20101109002559.GA30016@xxxxxxxxxxxxx>
References: <20101109002559.GA30016@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Mon, Nov 08, 2010 at 07:26:00PM -0500, Christoph Hellwig wrote:
> Since the move to the new truncate sequence we call xfs_setattr to
> truncate down excessively instanciated blocks.  As shown by the testcase
> in kernel.org BZ #22452 that doesn't work too well.  Due to the confusion
> of the internal inode size, and the VFS inode i_size it zeroes data that
> it shouldn't.
> But full blown truncate seems like overkill here.  We only instanciate
> delayed allocations in the write path, and given that we never released
> the iolock we can't have converted them to real allocations yet either.
> The only nasty case is pre-existing preallocation which we need to skip.
> The patch below does that by borrowing code from xfs_aops_discard_page.
> It does pass xfstests for 4k block filesystems and fixes the original
> bug.  I'm not quite sure if we could hit a corner case with smaller
> block sizes when parts of a page are preallocated and some not.

Seems likely - preallocated block past EOF are not unusual.

> That
> could be handled by looping around bmapi as long as we find extents
> for our range.  The path could probably also be refactored to share
> code with xfs_aops_discard_page.  And we probably need the ilock
> just as in that path, but I only got to that when almost through
> xfstests, and the day is over for me today, so let's just get the
> patch out for now.

Yes, definitely need the ilock - that's the only lock that provides
protection for the extent tree.

It looks to me that we need a general "discard delalloc blocks from
range" function - I'll write one (basically the guts of
xfs_aops_discard_page) and convert xfs_aops_discard_page() and this
code to use it....


Dave Chinner

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