linux-kernel - Re: [PATCH AUTOSEL 5.17 17/21] btrfs: reset last_reflink

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Date:   Thu, 31 Mar 2022 12:59:33 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     Filipe Manana <fdmanana@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Filipe Manana <fdmanana@...e.com>,
        David Sterba <dsterba@...e.com>, clm@...com, jbacik@...com,
        linux-btrfs@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 5.17 17/21] btrfs: reset last_reflink_trans after
 fsyncing inode

On Tue, Mar 29, 2022 at 10:59:33AM +0100, Filipe Manana wrote:
>On Mon, Mar 28, 2022 at 03:41:52PM -0400, Sasha Levin wrote:
>> From: Filipe Manana <fdmanana@...e.com>
>>
>> [ Upstream commit 23e3337faf73e5bb2610697977e175313d48acb0 ]
>>
>> When an inode has a last_reflink_trans matching the current transaction,
>> we have to take special care when logging its checksums in order to
>> avoid getting checksum items with overlapping ranges in a log tree,
>> which could result in missing checksums after log replay (more on that
>> in the changelogs of commit 40e046acbd2f36 ("Btrfs: fix missing data
>> checksums after replaying a log tree") and commit e289f03ea79bbc ("btrfs:
>> fix corrupt log due to concurrent fsync of inodes with shared extents")).
>> We also need to make sure a full fsync will copy all old file extent
>> items it finds in modified leaves, because they might have been copied
>> from some other inode.
>>
>> However once we fsync an inode, we don't need to keep paying the price of
>> that extra special care in future fsyncs done in the same transaction,
>> unless the inode is used for another reflink operation or the full sync
>> flag is set on it (truncate, failure to allocate extent maps for holes,
>> and other exceptional and infrequent cases).
>>
>> So after we fsync an inode reset its last_unlink_trans to zero. In case
>> another reflink happens, we continue to update the last_reflink_trans of
>> the inode, just as before. Also set last_reflink_trans to the generation
>> of the last transaction that modified the inode whenever we need to set
>> the full sync flag on the inode, just like when we need to load an inode
>> from disk after eviction.
>>
>> Signed-off-by: Filipe Manana <fdmanana@...e.com>
>> Signed-off-by: David Sterba <dsterba@...e.com>
>> Signed-off-by: Sasha Levin <sashal@...nel.org>
>
>What's the motivation to backport this to stable?
>
>It doesn't fix a bug or any regression, as far as I know at least.
>Or is it to make some other backport easier?

I wasn't sure if it's needed for completeness for the mentioned fixes,
so I took it. Can drop it if it's not needed.

-- 
Thanks,
Sasha