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Date:   Fri, 28 Jul 2023 22:00:53 +0200
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Subject: Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow_protnone
 fallout
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To:     Peter Xu <peterx@redhat.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        liubo <liubo254@huawei.com>,
        Matthew Wilcox <willy@infradead.org>,
        Hugh Dickins <hughd@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        John Hubbard <jhubbard@nvidia.com>,
        Mel Gorman <mgorman@techsingularity.net>
References: <20230727212845.135673-1-david@redhat.com>
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 <b647fd9a-3d75-625c-9f2c-dd3c251528c4@redhat.com> <ZMQcBWvjVUEBU6mF@x1n>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <ZMQcBWvjVUEBU6mF@x1n>
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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