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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-436dcc8f8dfsm146389445e9.0.2025.01.13.04.20.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 13 Jan 2025 04:20:24 -0800 (PST) Message-ID: <5c62bdbb-7a4e-4178-8c03-e84491d8d150@redhat.com> Date: Mon, 13 Jan 2025 13:20:23 +0100 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 0/2] mm: filemap: add filemap_grab_folios To: kalyazin@amazon.com, willy@infradead.org, pbonzini@redhat.com, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: michael.day@amd.com, jthoughton@google.com, michael.roth@amd.com, ackerleytng@google.com, graf@amazon.de, jgowans@amazon.com, roypat@amazon.co.uk, derekmn@amazon.com, nsaenz@amazon.es, xmarcalx@amazon.com References: <20250110154659.95464-1-kalyazin@amazon.com> <5608af05-0b7a-4e11-b381-8b57b701e316@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 10.01.25 19:54, Nikita Kalyazin wrote: > On 10/01/2025 17:01, David Hildenbrand wrote: >> On 10.01.25 16:46, Nikita Kalyazin wrote: >>> Based on David's suggestion for speeding up guest_memfd memory >>> population [1] made at the guest_memfd upstream call on 5 Dec 2024 [2], >>> this adds `filemap_grab_folios` that grabs multiple folios at a time. >>> >> >> Hi, > > Hi :) > >> >>> Motivation >>> >>> When profiling guest_memfd population and comparing the results with >>> population of anonymous memory via UFFDIO_COPY, I observed that the >>> former was up to 20% slower, mainly due to adding newly allocated pages >>> to the pagecache.  As far as I can see, the two main contributors to it >>> are pagecache locking and tree traversals needed for every folio.  The >>> RFC attempts to partially mitigate those by adding multiple folios at a >>> time to the pagecache. >>> >>> Testing >>> >>> With the change applied, I was able to observe a 10.3% (708 to 635 ms) >>> speedup in a selftest that populated 3GiB guest_memfd and a 9.5% (990 to >>> 904 ms) speedup when restoring a 3GiB guest_memfd VM snapshot using a >>> custom Firecracker version, both on Intel Ice Lake. >> >> Does that mean that it's still 10% slower (based on the 20% above), or >> were the 20% from a different micro-benchmark? > > Yes, it is still slower: > - isolated/selftest: 2.3% > - Firecracker setup: 8.9% > > Not sure why the values are so different though. I'll try to find an > explanation. The 2.3% looks very promising. > >>> >>> Limitations >>> >>> While `filemap_grab_folios` handles THP/large folios internally and >>> deals with reclaim artifacts in the pagecache (shadows), for simplicity >>> reasons, the RFC does not support those as it demonstrates the >>> optimisation applied to guest_memfd, which only uses small folios and >>> does not support reclaim at the moment. >> >> It might be worth pointing out that, while support for larger folios is >> in the works, there will be scenarios where small folios are unavoidable >> in the future (mixture of shared and private memory). >> >> How hard would it be to just naturally support large folios as well? > > I don't think it's going to be impossible. It's just one more dimension > that needs to be handled. `__filemap_add_folio` logic is already rather > complex, and processing multiple folios while also splitting when > necessary correctly looks substantially convoluted to me. So my idea > was to discuss/validate the multi-folio approach first before rolling > the sleeves up. We should likely try making this as generic as possible, meaning we'll support roughly what filemap_grab_folio() would have supported (e.g., also large folios). Now I find filemap_get_folios_contig() [thas is already used in memfd code], and wonder if that could be reused/extended fairly easily. -- Cheers, David / dhildenb