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Message-ID: <30d86a2d-4af2-d840-91be-2e68c73a07bd@redhat.com>
Date:   Tue, 1 Aug 2023 18:15:48 +0200
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Subject: Re: [PATCH v2 1/8] mm/gup: reintroduce FOLL_NUMA as
 FOLL_HONOR_NUMA_FAULT
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To:     Peter Xu <peterx@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, kvm@vger.kernel.org,
        linux-kselftest@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@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@suse.de>, Shuah Khan <shuah@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>, stable@vger.kernel.org
References: <20230801124844.278698-1-david@redhat.com>
 <20230801124844.278698-2-david@redhat.com> <ZMkpM95vdc9wgs9T@x1n>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <ZMkpM95vdc9wgs9T@x1n>
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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