From: Brian Foster <bfoster@redhat.com>
To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: linux-fsdevel@vger.kernel.org, linux-afs@lists.infradead.org,
linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org,
linux-mm@kvack.org, Hugh Dickins <hughd@google.com>,
linux-kernel@vger.kernel.org, fstests@vger.kernel.org
Subject: Re: [PATCH 1/5] truncate: Zero bytes after 'oldsize' if we're expanding the file
Date: Fri, 3 Feb 2023 08:00:16 -0500 [thread overview]
Message-ID: <Y90FYG+tNtBIl62S@bfoster> (raw)
In-Reply-To: <20230202204428.3267832-2-willy@infradead.org>
On Thu, Feb 02, 2023 at 08:44:23PM +0000, Matthew Wilcox (Oracle) wrote:
> POSIX requires that "If the file size is increased, the extended area
> shall appear as if it were zero-filled". It is possible to use mmap to
> write past EOF and that data will become visible instead of zeroes.
> This fixes the problem for the filesystems which simply call
> truncate_setsize(). More complex filesystems will need their own
> patches.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
> mm/truncate.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/mm/truncate.c b/mm/truncate.c
> index 7b4ea4c4a46b..cebfc5415e9a 100644
> --- a/mm/truncate.c
> +++ b/mm/truncate.c
> @@ -763,9 +763,12 @@ void truncate_setsize(struct inode *inode, loff_t newsize)
> loff_t oldsize = inode->i_size;
>
> i_size_write(inode, newsize);
> - if (newsize > oldsize)
> + if (newsize > oldsize) {
> pagecache_isize_extended(inode, oldsize, newsize);
> - truncate_pagecache(inode, newsize);
> + truncate_pagecache(inode, oldsize);
> + } else {
> + truncate_pagecache(inode, newsize);
> + }
I don't think this alone quite addresses the problem. Looking at ext4
for example, if the eof page is dirty and writeback occurs between the
i_size update (because writeback also zeroes the post-eof portion of the
page) and the truncate_setsize() call, we end up with pagecache
inconsistency because pagecache truncate doesn't dirty the page it
zeroes.
So for example, with this series plus a nefariously placed
filemap_flush() in ext4_setattr():
# xfs_io -fc "truncate 1" -c "mmap 0 1k" -c "mwrite 0 10" -c "truncate 5" -c "mread -v 0 5" /mnt/file
00000000: 58 00 00 00 00 X....
# umount /mnt/; mount <dev> /mnt/
# xfs_io -c "mmap 0 1k" -c "mread -v 0 5" /mnt/file
00000000: 58 58 58 58 58 XXXXX
Brian
> }
> EXPORT_SYMBOL(truncate_setsize);
>
> --
> 2.35.1
>
>
next prev parent reply other threads:[~2023-02-03 12:59 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-02 20:44 [PATCH 0/5] Fix a minor POSIX conformance problem Matthew Wilcox (Oracle)
2023-02-02 20:44 ` [PATCH 1/5] truncate: Zero bytes after 'oldsize' if we're expanding the file Matthew Wilcox (Oracle)
2023-02-03 13:00 ` Brian Foster [this message]
2023-02-03 15:07 ` Matthew Wilcox
2023-02-02 20:44 ` [PATCH 2/5] ext4: " Matthew Wilcox (Oracle)
2023-02-02 20:44 ` [PATCH 3/5] tmpfs: " Matthew Wilcox (Oracle)
2023-02-02 20:44 ` [PATCH 4/5] afs: " Matthew Wilcox (Oracle)
2023-02-02 20:44 ` [PATCH 5/5] btrfs: " Matthew Wilcox (Oracle)
2023-02-02 20:44 ` [PATCH 6/5] generic: test ftruncate zeroes bytes after EOF Matthew Wilcox (Oracle)
2023-02-03 11:57 ` Johannes Thumshirn
2023-02-03 13:13 ` Matthew Wilcox
2023-02-03 16:35 ` Darrick J. Wong
2023-02-02 23:08 ` [PATCH 0/5] Fix a minor POSIX conformance problem Andreas Dilger
2023-02-03 13:21 ` Matthew Wilcox
2023-02-03 16:23 ` David Laight
2023-02-03 16:29 ` Matthew Wilcox
2023-02-27 13:49 ` [PATCH 4/5] afs: Zero bytes after 'oldsize' if we're expanding the file David Howells
2023-02-27 14:09 ` Matthew Wilcox
2023-02-27 14:20 ` David Howells
2023-02-27 14:49 ` David Howells
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