Re: [PATCH AUTOSEL 6.0 38/46] btrfs: introduce BTRFS_QGROUP_RUNTIME_FLAG_CANCEL_RESCAN

[Sasha Levin]

On Thu, Oct 13, 2022 at 07:12:10AM +0800, Qu Wenruo wrote:
> On 2022/10/12 20:56, David Sterba wrote:
> > On Tue, Oct 11, 2022 at 10:50:06AM -0400, Sasha Levin wrote:
> > > From: Qu Wenruo <wqu@xxxxxxxx>
> > >
> > > [ 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.
>
> Considering the qgroup still lacks a way to handle large subvolume drop,
> and a lot of things can mark qgroup inconsistent halfway, I think
> backporting this patch itself is not that bad.
>
> The problem is, why only backporting this one?
>
> To me, it would make more sense to backport either all or none.
>
> Sure, if we can cancel rescan it's an improvement, but rescan itself is
> already relatively cheap compared to other qgroup operations.
> Thus I prefer to backport all the qgroup patches.

I'll drop this one and happily take a series if you want to send one
out.

--
Thanks,
Sasha