linux-ext4 - Re: [PATCH] ext4: fix bigalloc cluster freeing when hole punching under load

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Message-ID: <20190301050009.GA8234@mit.edu>
Date:   Fri, 1 Mar 2019 00:00:09 -0500
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Eric Whitney <enwlinux@...il.com>
CC:     <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] ext4: fix bigalloc cluster freeing when hole punching
 under load

On Wed, Feb 27, 2019 at 05:02:04PM -0500, Eric Whitney wrote:
> Ext4 may not free clusters correctly when punching holes in bigalloc
> file systems under high load conditions.  If it's not possible to
> extend and restart the journal in ext4_ext_rm_leaf() when preparing to
> remove blocks from a punched region, a retry of the entire punch
> operation is triggered in ext4_ext_remove_space().  This causes a
> partial cluster to be set to the first cluster in the extent found to
> the right of the punched region.  However, if the punch operation
> prior to the retry had made enough progress to delete one or more
> extents and a partial cluster candidate for freeing had already been
> recorded, the retry would overwrite the partial cluster.  The loss of
> this information makes it impossible to correctly free the original
> partial cluster in all cases.
> 
> This bug can cause generic/476 to fail when run as part of
> xfstests-bld's bigalloc and bigalloc_1k test cases.  The failure is
> reported when e2fsck detects bad iblocks counts greater than expected
> in units of whole clusters and also detects a number of negative block
> bitmap differences equal to the iblocks discrepancy in cluster units.
> 
> Signed-off-by: Eric Whitney <enwlinux@...il.com>

Thanks, applied.

							- Ted