linux-kernel - Re: [PATCH v2 3/8] kvm: explicitly set FOLL_HONOR_NUMA_FAULT in hva_to_pfn

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Message-ID: <cc408a90-9044-54cd-04a5-5977ffe8b24b@redhat.com>
Date:   Wed, 2 Aug 2023 17:29:56 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
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>, Peter Xu <peterx@...hat.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>
Subject: Re: [PATCH v2 3/8] kvm: explicitly set FOLL_HONOR_NUMA_FAULT in
 hva_to_pfn_slow()

On 02.08.23 17:27, Mel Gorman wrote:
> On Tue, Aug 01, 2023 at 02:48:39PM +0200, David Hildenbrand wrote:
>> KVM is *the* case we know that really wants to honor NUMA hinting falls.
>> As we want to stop setting FOLL_HONOR_NUMA_FAULT implicitly, set
>> FOLL_HONOR_NUMA_FAULT whenever we might obtain pages on behalf of a VCPU
>> to map them into a secondary MMU, and add a comment why.
>>
>> Do that unconditionally in hva_to_pfn_slow() when calling
>> get_user_pages_unlocked().
>>
>> kvmppc_book3s_instantiate_page(), hva_to_pfn_fast() and
>> gfn_to_page_many_atomic() are similarly used to map pages into a
>> secondary MMU. However, FOLL_WRITE and get_user_page_fast_only() always
>> implicitly honor NUMA hinting faults -- as documented for
>> FOLL_HONOR_NUMA_FAULT -- so we can limit this change to a single location
>> for now.
>>
>> Don't set it in check_user_page_hwpoison(), where we really only want to
>> check if the mapped page is HW-poisoned.
>>
>> We won't set it for other KVM users of get_user_pages()/pin_user_pages()
>> * arch/powerpc/kvm/book3s_64_mmu_hv.c: not used to map pages into a
>>    secondary MMU.
>> * arch/powerpc/kvm/e500_mmu.c: only used on shared TLB pages with userspace
>> * arch/s390/kvm/*: s390x only supports a single NUMA node either way
>> * arch/x86/kvm/svm/sev.c: not used to map pages into a secondary MMU.
>>
>> This is a preparation for making FOLL_HONOR_NUMA_FAULT no longer
>> implicitly be set by get_user_pages() and friends.
>>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> Seems sane but I don't know KVM well enough to know if this is the only
> relevant case so didn't ack.

Makes sense, some careful eyes from KVM people would be appreciated.

At least from kvm_main.c POV, I'm pretty confident that that's it.

-- 
Cheers,

David / dhildenb