Re: [PATCH v2 1/8] mm/gup: reintroduce FOLL_NUMA as FOLL_HONOR_NUMA_FAULT

[David Hildenbrand <david@redhat.com>]

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@redhat.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