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

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Message-ID: <ZMQcBWvjVUEBU6mF@x1n>
Date:   Fri, 28 Jul 2023 15:50:29 -0400
From:   Peter Xu <peterx@...hat.com>
To:     David Hildenbrand <david@...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 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).

Thanks,

-- 
Peter Xu