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

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Date:   Fri, 28 Jul 2023 13:50:03 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
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 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.

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.

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.

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

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.

                 Linus