linux-ext4 - Re: [PATCH 1/2] ext4: Fix SEEK

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Date:   Sat, 20 May 2017 00:14:00 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org, stable@...r.kernel.org,
        Zheng Liu <wenqing.lz@...bao.com>
Subject: Re: [PATCH 1/2] ext4: Fix SEEK_HOLE

On Thu, May 18, 2017 at 12:49:41PM +0200, Jan Kara wrote:
> Currently, SEEK_HOLE implementation in ext4 may both return that there's
> a hole at some offset although that offset already has data and skip
> some holes during a search for the next hole. The first problem is
> demostrated by:
> 
> xfs_io -c "falloc 0 256k" -c "pwrite 0 56k" -c "seek -h 0" file
> wrote 57344/57344 bytes at offset 0
> 56 KiB, 14 ops; 0.0000 sec (2.054 GiB/sec and 538461.5385 ops/sec)
> Whence	Result
> HOLE	0
> 
> Where we can see that SEEK_HOLE wrongly returned offset 0 as containing
> a hole although we have written data there. The second problem can be
> demonstrated by:
> 
> xfs_io -c "falloc 0 256k" -c "pwrite 0 56k" -c "pwrite 128k 8k"
>        -c "seek -h 0" file
> 
> wrote 57344/57344 bytes at offset 0
> 56 KiB, 14 ops; 0.0000 sec (1.978 GiB/sec and 518518.5185 ops/sec)
> wrote 8192/8192 bytes at offset 131072
> 8 KiB, 2 ops; 0.0000 sec (2 GiB/sec and 500000.0000 ops/sec)
> Whence	Result
> HOLE	139264
> 
> Where we can see that hole at offsets 56k..128k has been ignored by the
> SEEK_HOLE call.
> 
> The underlying problem is in the ext4_find_unwritten_pgoff() which is
> just buggy. In some cases it fails to update returned offset when it
> finds a hole (when no pages are found or when the first found page has
> higher index than expected), in some cases conditions for detecting hole
> are just missing (we fail to detect a situation where indices of
> returned pages are not contiguous).
> 
> Fix ext4_find_unwritten_pgoff() to properly detect non-contiguous page
> indices and also handle all cases where we got less pages then expected
> in one place and handle it properly there.
> 
> CC: stable@...r.kernel.org
> Fixes: c8c0df241cc2719b1262e627f999638411934f60
> CC: Zheng Liu <wenqing.lz@...bao.com>
> Signed-off-by: Jan Kara <jack@...e.cz>

Thanks, applied.

					- Ted