linux-ext4 - Re: [PATCH v3] ext4: Fix occasional generic/418 failure

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Message-ID: <YIHh/wfyxLadZYGD@mit.edu>
Date:   Thu, 22 Apr 2021 16:52:15 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org, Eric Whitney <enwlinux@...il.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v3] ext4: Fix occasional generic/418 failure

On Thu, Apr 15, 2021 at 05:54:17PM +0200, Jan Kara wrote:
> Eric has noticed that after pagecache read rework, generic/418 is
> occasionally failing for ext4 when blocksize < pagesize. In fact, the
> pagecache rework just made hard to hit race in ext4 more likely. The
> problem is that since ext4 conversion of direct IO writes to iomap
> framework (commit 378f32bab371), we update inode size after direct IO
> write only after invalidating page cache. Thus if buffered read sneaks
> at unfortunate moment like:
> 
> CPU1 - write at offset 1k                       CPU2 - read from offset 0
> iomap_dio_rw(..., IOMAP_DIO_FORCE_WAIT);
>                                                 ext4_readpage();
> ext4_handle_inode_extension()
> 
> the read will zero out tail of the page as it still sees smaller inode
> size and thus page cache becomes inconsistent with on-disk contents with
> all the consequences.
> 
> Fix the problem by moving inode size update into end_io handler which
> gets called before the page cache is invalidated.
> 
> Reported-and-tested-by: Eric Whitney <enwlinux@...il.com>
> Fixes: 378f32bab371 ("ext4: introduce direct I/O write using iomap infrastructure")
> CC: stable@...r.kernel.org
> Signed-off-by: Jan Kara <jack@...e.cz>

Thanks, applied.

					- Ted