linux-kernel - Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow

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Message-ID: <69a5f457-63b6-2d4f-e5c0-4b3de1e6c9f1@redhat.com>
Date:   Fri, 28 Jul 2023 23:02:46 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        liubo <liubo254@...wei.com>,
        Matthew Wilcox <willy@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow_protnone
 fallout

On 28.07.23 22:50, Linus Torvalds wrote:
> On Fri, 28 Jul 2023 at 13:33, David Hildenbrand <david@...hat.com> wrote:
>>
>> So would you rather favor a FOLL_NUMA that has to be passed from the
>> outside by selected callers or a FOLL_NUMA that is set on the GUP path
>> unconditionally (but left clear for follow_page())?
> 
> I'd rather see the FOLL_NUMA that has to be set by odd cases, and that
> is never set by any sane user.

Thanks!

> 
> And it should not be called FOLL_NUMA. It should be called something
> else. Because *not* having it doesn't disable following pages across
> NUMA boundaries, and the name is actively misleading.
> 
> It sounds like what KVM actually wants is a "Do NOT follow NUMA pages,
> I'll force a page fault".
> 
> And the fact that KVM wants a fault for NUMA pages shouldn't mean that
> others - who clearly cannot care - get that insane behavior by
> default.

For KVM it represents actual CPU access. To map these pages into the VM 
MMU we have to look them up from the process -- in the context of the 
faulting CPU. So it makes a lot of sense for KVM. (which is also where 
autonuma gets heavily used)

> 
> The name should reflect that, instead of being the misleading mess of
> FOLL_FORCE and bad naming that it is now.
> 
> So maybe it can be called "FOLL_HONOR_NUMA_FAULT" or something, to
> make it clear that it's the *opposite* of FOLL_FORCE, and that it
> honors the NUMA faulting that nobody should care about.

Naming sounds much better to me.

> 
> Then the KVM code can have a big comment about *why* it sets that bit.

Yes.

> 
> Hmm? Can we please aim for something that is understandable and
> documented? No odd implicit rules. No "force NUMA fault even when it
> makes no sense". No tie-in with FOLL_FORCE.

I mean, I messed all that FOLL_NUMA handling up because I was very 
confused. So I'm all for better documentation.


Can we get a simple revert in first (without that FOLL_FORCE special 
casing and ideally with a better name) to handle stable backports, and 
I'll follow-up with more documentation and letting GUP callers pass in 
that flag instead?

That would help a lot. Then we also have more time to let that "move it 
to GUP callers" mature a bit in -next, to see if we find any surprises?

-- 
Cheers,

David / dhildenb