Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <cc408a90-9044-54cd-04a5-5977ffe8b24b@redhat.com>
Date:   Wed, 2 Aug 2023 17:29:56 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.13.0
Subject: Re: [PATCH v2 3/8] kvm: explicitly set FOLL_HONOR_NUMA_FAULT in
 hva_to_pfn_slow()
Content-Language: en-US
To:     Mel Gorman <mgorman@techsingularity.net>
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>, Peter Xu <peterx@redhat.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>
References: <20230801124844.278698-1-david@redhat.com>
 <20230801124844.278698-4-david@redhat.com>
 <20230802152702.wamtroy3zm7nbtvs@techsingularity.net>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20230802152702.wamtroy3zm7nbtvs@techsingularity.net>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

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@redhat.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