linux-ext4 - Re: [PATCH] ext4: fix race aio-dio vs freeze

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Date:	Tue, 24 Nov 2015 14:24:21 +0100
From:	Jan Kara <jack@...e.cz>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:	linux-ext4@...r.kernel.org, jack@...e.cz, tytso@....edu,
	xfs@....sgi.com, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] ext4: fix race aio-dio vs freeze_fs

On Mon 23-11-15 20:02:48, Dmitry Monakhov wrote:
> After freeze_fs was revoked (from Jan Kara) pages's write-back completion
> is deffered before unwritten conversion, so explicit flush_unwritten_io()
> was removed here: c724585b62411
> But we still may face deferred conversion for aio-dio case
> # Trivial testcase
> for ((i=0;i<60;i++));do fsfreeze -f /mnt ;sleep 1;fsfreeze -u /mnt;done &
> fio --bs=4k --ioengine=libaio --iodepth=128 --size=1g --direct=1 \
>     --runtime=60 --filename=/mnt/file --name=rand-write --rw=randwrite
> NOTE: Sane testcase should be integrated to xfstests, but it requires
> changes in common/* code, so let's use this this test at the moment.
> 
> In order to fix this race we have to guard journal transaction with explicit
> sb_{start,end}_intwrite()  as we do with ext4_evict_inode here:8e8ad8a5

Well, this problem seems to suggest that we have the freeze protection for
AIO writes wrong. We should call file_end_write() from aio_complete() and
not from aio_run_iocb()... I believe XFS and other filesystems may have
problems with this as well (CCed). Attached patch (so far only compile
tested since my test machine is pondering on something else) should fix
this.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

View attachment "0001-aio-Fix-freeze-protection-of-aio-writes.patch" of type "text/x-patch" (2143 bytes)