From: Al Viro <viro@ZenIV.linux.org.uk>
To: Dave Chinner <david@fromorbit.com>
Cc: linux-fsdevel@vger.kernel.org, Brian Foster <bfoster@redhat.com>,
xfs@oss.sgi.com, Dave Chinner <dchinner@redhat.com>
Subject: Re: fs corruption exposed by "xfs: increase prealloc size to double that of the previous extent"
Date: Mon, 17 Mar 2014 00:29:18 +0000 [thread overview]
Message-ID: <20140317002918.GT18016@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20140317001130.GA7072@dastard>
On Mon, Mar 17, 2014 at 11:11:30AM +1100, Dave Chinner wrote:
> Yes, we've known about this since 2011. Right - that's a long
> standing problem, and one I've never been able to isolate and so
> reproduce with any luck. It can only be reproduced when you use mmap
> and direct IO on the same file, and every time I've added debug to
> find out where the tail block corruption was being introduced, the
> data corruption goes away. It behaves just like a race condition....
See downthread. And I would be *very* surprised if it was a race -
don't forget the msync() done before that write().
I think I know what's going on - O_DIRECT write starting a bit before
EOF on a file with the last extent that can be grown. It fills
a buffer_head with b_size extending quite a bit past the EOF; the
blocks are really allocated. What causes the problem is that we
have the flags set for the *first* block. IOW, buffer_new(bh) is
false - the first block has already been allocated. And for
direct-io.c it means "no zeroing the tail of the last block".
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WARNING: multiple messages have this Message-ID (diff)
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Dave Chinner <david@fromorbit.com>
Cc: Brian Foster <bfoster@redhat.com>,
linux-fsdevel@vger.kernel.org, Dave Chinner <dchinner@redhat.com>,
xfs@oss.sgi.com
Subject: Re: fs corruption exposed by "xfs: increase prealloc size to double that of the previous extent"
Date: Mon, 17 Mar 2014 00:29:18 +0000 [thread overview]
Message-ID: <20140317002918.GT18016@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20140317001130.GA7072@dastard>
On Mon, Mar 17, 2014 at 11:11:30AM +1100, Dave Chinner wrote:
> Yes, we've known about this since 2011. Right - that's a long
> standing problem, and one I've never been able to isolate and so
> reproduce with any luck. It can only be reproduced when you use mmap
> and direct IO on the same file, and every time I've added debug to
> find out where the tail block corruption was being introduced, the
> data corruption goes away. It behaves just like a race condition....
See downthread. And I would be *very* surprised if it was a race -
don't forget the msync() done before that write().
I think I know what's going on - O_DIRECT write starting a bit before
EOF on a file with the last extent that can be grown. It fills
a buffer_head with b_size extending quite a bit past the EOF; the
blocks are really allocated. What causes the problem is that we
have the flags set for the *first* block. IOW, buffer_new(bh) is
false - the first block has already been allocated. And for
direct-io.c it means "no zeroing the tail of the last block".
next prev parent reply other threads:[~2014-03-17 0:29 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-03-15 21:02 fs corruption exposed by "xfs: increase prealloc size to double that of the previous extent" Al Viro
2014-03-15 21:02 ` Al Viro
2014-03-16 2:21 ` Al Viro
2014-03-16 2:39 ` Al Viro
2014-03-16 2:39 ` Al Viro
2014-03-16 20:56 ` Al Viro
2014-03-16 20:56 ` Al Viro
2014-03-17 1:36 ` Dave Chinner
2014-03-17 1:36 ` Dave Chinner
2014-03-17 2:43 ` Dave Chinner
2014-03-18 1:16 ` Dave Chinner
2014-03-17 0:11 ` Dave Chinner
2014-03-17 0:11 ` Dave Chinner
2014-03-17 0:29 ` Al Viro [this message]
2014-03-17 0:29 ` Al Viro
2014-03-17 1:28 ` Al Viro
2014-03-17 1:38 ` Al Viro
2014-03-17 1:41 ` Dave Chinner
2014-03-17 1:41 ` Dave Chinner
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