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

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Message-ID: <cd4823fb-b00d-1029-cbd8-c23789c28858@redhat.com>
Date:   Fri, 28 Jul 2023 22:00:53 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow_protnone
 fallout

On 28.07.23 21:50, Peter Xu wrote:
> On Fri, Jul 28, 2023 at 09:40:54PM +0200, David Hildenbrand wrote:
>> Hmm. So three alternatives I see:
>>
>> 1) Use FOLL_FORCE in follow_page() to unconditionally disable protnone
>>     checks. Alternatively, have an internal FOLL_NO_PROTNONE flag if we
>>     don't like that.
>>
>> 2) Revert the commit and reintroduce unconditional FOLL_NUMA without
>>     FOLL_FORCE.
>>
>> 3) Have a FOLL_NUMA that callers like KVM can pass.
> 
> I'm afraid 3) means changing numa balancing to opt-in, probably no-go for
> any non-kvm gup users as that could start to break there, even if making
> smaps/follow_page happy again.
> 
> I keep worrying 1) on FOLL_FORCE abuse.
> 
> So I keep my vote on 2).

Linus had a point that we can nowadays always set FOLL_NUMA, even with 
FOLL_FORCE.

... so maybe we want to let GUP always set FOLL_NUMA (even with 
FOLL_FORCE) and follow_page() never set FOLL_NUMA.

That would at least decouple FOLL_NUMA from FOLL_FORCE.

-- 
Cheers,

David / dhildenb