linux-kernel - Re: [PATCH v2 1/8] mm/gup: reintroduce FOLL_NUMA as FOLL_HONOR_NUMA

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Date:   Tue, 1 Aug 2023 19:09:55 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, kvm@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...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@...e.de>, Shuah Khan <shuah@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/8] mm/gup: reintroduce FOLL_NUMA as
 FOLL_HONOR_NUMA_FAULT

On 01.08.23 19:04, Peter Xu wrote:
> On Tue, Aug 01, 2023 at 06:15:48PM +0200, David Hildenbrand wrote:
>> On 01.08.23 17:48, Peter Xu wrote:
>>> On Tue, Aug 01, 2023 at 02:48:37PM +0200, David Hildenbrand wrote:
>>>> @@ -2240,6 +2244,12 @@ static bool is_valid_gup_args(struct page **pages, int *locked,
>>>>    		gup_flags |= FOLL_UNLOCKABLE;
>>>>    	}
>>>> +	/*
>>>> +	 * For now, always trigger NUMA hinting faults. Some GUP users like
>>>> +	 * KVM really require it to benefit from autonuma.
>>>> +	 */
>>>> +	gup_flags |= FOLL_HONOR_NUMA_FAULT;
>>>
>>> Since at it, do we want to not set it for FOLL_REMOTE, which still sounds
>>> like a good thing to have?
>>
>> I thought about that, but decided against making that patch here more
>> complicated to eventually rip it again all out in #4.
> 
> I thought that was the whole point of having patch 4 separate, because we
> should assume patch 4 may not exist in (at least) some trees, so I ignored
> patch 4 when commenting here, and we should not assume it's destined to be
> removed here.

For me, the goal of this patch is to bring it *as close as possible* to 
the previous state as before, so we can backport it to stable without 
too many surprises (effectively, only a handful of FOLL_FORCE/ptrace 
user will get a different behavior).

I could add a separate patch that does the FOLL_REMOTE thing, but then, 
maybe we should do that if patch #4 runs into real trouble :)

But no strong opinion if this is what everybody wants in this patch.

> 
>>
>> I fully agree that FOLL_REMOTE does not make too much sense, but let's
>> rather keep it simple for this patch.
> 
> It's fine - I suppose this patch fixes whatever we're aware of that's
> broken with FOLL_NUMA's removal, so it can even be anything on top when
> needed.  I assume I'm happy to ack this with/without that change, then:
> 
> Acked-by: Peter Xu <peterx@...hat.com>

Thanks!

> 
> PS: I still hope that the other oneliner can be squashed here directly; it
> literally changes exact the same line above so reading this patch alone can
> be affected.  You said there you didn't want the commit message to be too
> long here, but this is definitely not long at all!  I bet you have similar
> understanding to me on defining "long commit message". :) I'd never worry
> that.  Your call.

No strong opinion, it just felt cleaner to not start adding what I have 
in that separate patch commit message in here.

-- 
Cheers,

David / dhildenb