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

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Message-ID: <aab2cb18-67e1-e55b-57e0-7e0ac90f3da3@redhat.com>
Date:   Fri, 28 Jul 2023 21:52:11 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     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 21:39, Peter Xu wrote:
> Hi, Linus,
> 
> On Fri, Jul 28, 2023 at 09:18:45AM -0700, Linus Torvalds wrote:
>> The original reason for FOLL_NUMA simply does not exist any more. We
>> know exactly when a page is marked for NUMA faulting, and we should
>> simply *ignore* it for GUP and follow_page().
>>
>> I think we should treat a NUMA-faulting page as just being present
>> (and not NUMA-fault it).
> 
> But then does it means that any gup-only user will have numa balancing
> completely disabled?  Since as long as the page will only be accessed by
> GUP, the numa balancing will never trigger anyway..  I think KVM is
> manipulating guest pages just like that.  Not sure whether it means it'll
> void the whole numa effort there.
> 
> If we allow that GUP from happening (taking protnone as present) I assume
> it'll also stop any further numa balancing on this very page to trigger
> too, because even if some page fault handler triggered on this protnone
> page later that is not GUP anymore, when it wants to migrate the page to
> the other numa node it'll see someone is holding a reference on it already,
> and then we should give up the balancing.
> 
> So to me FOLL_NUMA (or any identifier like it.. just to describe the
> caller's need; some may really just want to fetch the pfn/page) still makes
> sense.  But maybe I totally misunderstood above..

Yes, I agree, took me a bit longer to realize (being a KVM developer :) 
... I'm really ready for the weekend).

So if this series is not acceptable then better revert that commit -- or 
let callers like KVM specify FOLL_NUMA.

-- 
Cheers,

David / dhildenb