linux-kernel - Re: [PATCH AUTOSEL 6.0 38/46] btrfs: introduce BTRFS_QGROUP_RUNTIME_FLAG_CANCEL

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Message-ID: <20221012125648.GX13389@suse.cz>
Date:   Wed, 12 Oct 2022 14:56:48 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Qu Wenruo <wqu@...e.com>, David Sterba <dsterba@...e.com>,
        clm@...com, josef@...icpanda.com, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 6.0 38/46] btrfs: introduce
 BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN

On Tue, Oct 11, 2022 at 10:50:06AM -0400, Sasha Levin wrote:
> From: Qu Wenruo <wqu@...e.com>
> 
> [ Upstream commit e562a8bdf652b010ce2525bcf15d145c9d3932bf ]
> 
> Introduce a new runtime flag, BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN,
> which will inform qgroup rescan to cancel its work asynchronously.
> 
> This is to address the window when an operation makes qgroup numbers
> inconsistent (like qgroup inheriting) while a qgroup rescan is running.
> 
> In that case, qgroup inconsistent flag will be cleared when qgroup
> rescan finishes.
> But we changed the ownership of some extents, which means the rescan is
> already meaningless, and the qgroup inconsistent flag should not be
> cleared.
> 
> With the new flag, each time we set INCONSISTENT flag, we also set this
> new flag to inform any running qgroup rescan to exit immediately, and
> leaving the INCONSISTENT flag there.
> 
> The new runtime flag can only be cleared when a new rescan is started.

Qu, does this patch make sense for stable on itself? It was part of a
series adding some new flags and the sysfs knob.  As I read it there's a
case where it can affect how the rescan is done and that it can be
cancelled but still am not sure if it's worth the backport.