linux-kernel - Re: [PATCH AUTOSEL 5.17 12/21] btrfs: don't advance offset for compressed bios in btrfs_csum_one

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Message-ID: <YkXdtBfNbH2/oP5z@sashalap>
Date:   Thu, 31 Mar 2022 12:58:28 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     Omar Sandoval <osandov@...ndov.com>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Omar Sandoval <osandov@...com>,
        Josef Bacik <josef@...icpanda.com>,
        Nikolay Borisov <nborisov@...e.com>,
        David Sterba <dsterba@...e.com>, clm@...com, jbacik@...com,
        linux-btrfs@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 5.17 12/21] btrfs: don't advance offset for
 compressed bios in btrfs_csum_one_bio()

On Tue, Mar 29, 2022 at 12:57:23PM -0700, Omar Sandoval wrote:
>On Mon, Mar 28, 2022 at 03:41:47PM -0400, Sasha Levin wrote:
>> From: Omar Sandoval <osandov@...com>
>>
>> [ Upstream commit e331f6b19f8adde2307588bb325ae5de78617c20 ]
>>
>> btrfs_csum_one_bio() loops over each filesystem block in the bio while
>> keeping a cursor of its current logical position in the file in order to
>> look up the ordered extent to add the checksums to. However, this
>> doesn't make much sense for compressed extents, as a sector on disk does
>> not correspond to a sector of decompressed file data. It happens to work
>> because:
>>
>> 1) the compressed bio always covers one ordered extent
>> 2) the size of the bio is always less than the size of the ordered
>>    extent
>>
>> However, the second point will not always be true for encoded writes.
>>
>> Let's add a boolean parameter to btrfs_csum_one_bio() to indicate that
>> it can assume that the bio only covers one ordered extent. Since we're
>> already changing the signature, let's get rid of the contig parameter
>> and make it implied by the offset parameter, similar to the change we
>> recently made to btrfs_lookup_bio_sums(). Additionally, let's rename
>> nr_sectors to blockcount to make it clear that it's the number of
>> filesystem blocks, not the number of 512-byte sectors.
>>
>> Reviewed-by: Josef Bacik <josef@...icpanda.com>
>> Reviewed-by: Nikolay Borisov <nborisov@...e.com>
>> Signed-off-by: Omar Sandoval <osandov@...com>
>> Signed-off-by: David Sterba <dsterba@...e.com>
>> Signed-off-by: Sasha Levin <sashal@...nel.org>
>> ---
>>  fs/btrfs/compression.c |  2 +-
>>  fs/btrfs/ctree.h       |  2 +-
>>  fs/btrfs/file-item.c   | 37 +++++++++++++++++--------------------
>>  fs/btrfs/inode.c       |  8 ++++----
>>  4 files changed, 23 insertions(+), 26 deletions(-)
>
>Hi, Sasha,
>
>This patch doesn't fix a real bug, so it should be dropped from both
>5.16 and 5.17.

I'll drop it, thanks.

-- 
Thanks,
Sasha