linux-kernel - Re: [PATCH 5.10 096/126] KVM: x86/mmu: Use atomic ops to set SPTEs in TDP MMU map

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Message-ID: <b2b05936-f545-9272-714b-845d54fa78eb@redhat.com>
Date:   Tue, 6 Apr 2021 22:43:52 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sasha Levin <sashal@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Peter Feiner <pfeiner@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH 5.10 096/126] KVM: x86/mmu: Use atomic ops to set SPTEs in
 TDP MMU map

On 06/04/21 21:44, Sasha Levin wrote:
> On Tue, Apr 06, 2021 at 08:28:27PM +0200, Paolo Bonzini wrote:
>> If a patch doesn't (more or less trivially) apply, the maintainer 
>> should take action.  Distro maintainers can also jump in and post the 
>> backport to subsystem mailing lists.  If the stable kernel loses a 
>> patch because a maintainer doesn't have the time to do a backport, 
>> it's not the end of the world.
> 
> This quickly went from a "world class" to "fuck it".

Known bugs are better than unknown bugs.  If something is reported on 
4.19 and the stable@ backports were only done up to 5.4 because the 
backport was a bit more messy, it's okay.  If a user comes up with a 
weird bug that I've never seen and that it's caused by a botched 
backport, it's much worse.

In this specific case we're talking of 5.10; but in many cases users of 
really old kernels, let's say 4.4-4.9 at this point, won't care about 
having *all* bugfixes.  Some newer (and thus more buggy) features may be 
so distant from the mainline in those kernels, or so immature, that no 
one will consider them while staying on such an old kernel.

Again, kernel necrophilia pays my bills, so I have some experience there. :)

> It's understandable that maintainers don't have all the time in the
> world for this, but are you really asking not to backport fixes to
> stable trees because you don't have the time for it and don't want
> anyone else to do it instead?

If a bug is serious I *will* do the backport; I literally did this 
specific backport on the first working day after the failure report. 
But not all bugs are created equal and neither are all stable@...rthy 
bugs.  I try to use the "Fixes" tag correctly, but sometimes a bug that 
*technically* is 10-years-old may not be worthwhile or even possible to 
fix even in 5.4.

That said... one thing that would be really, really awesome would be a 
website where you navigate a Linux checkout and for each directory you 
can choose to get a list of stable patches that were Cc-ed to stable@ 
and failed to apply.  A pipedream maybe, but also a great internship 
project. :)

Paolo

> Maybe consider designating someone who knows the subsystem well and does
> have time for this?