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 18:15:48 +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 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 fully agree that FOLL_REMOTE does not make too much sense, but let's 
rather keep it simple for this patch.


Thanks!

> 
> Other than that, looks good here.
> 
> Side note: when I was looking at the flags again just to check the
> interactions over numa balancing, I found FOLL_NOFAULT and I highly suspect
> that's not needed, instead it just wants to use follow_page[_mask]() with
> some proper gup flags passed over.. but that's off topic.

Be prepared for my proposal of removing foll_flags from follow_page() ;)

(accompanied by a proper documentation)

Especially as we have FOLL_PIN users of FOLL_NOFAULT, follow_page() is a 
bad fit.

-- 
Cheers,

David / dhildenb