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[2003:d8:2f44:3e00:9fca:7d89:a265:56f3]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45a1c6be10esm19816845e9.3.2025.08.14.04.58.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 14 Aug 2025 04:58:35 -0700 (PDT) Message-ID: <226e21a5-8a8e-43a0-bf5f-12a761f49a27@redhat.com> Date: Thu, 14 Aug 2025 13:58:34 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 6/7] mm: memory: add mTHP support for wp To: Vernon Yang , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, glider@google.com, elver@google.com, dvyukov@google.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, muchun.song@linux.dev, osalvador@suse.de, shuah@kernel.org, richardcochran@gmail.com Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.or References: <20250814113813.4533-1-vernon2gm@gmail.com> <20250814113813.4533-7-vernon2gm@gmail.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: F2C3B1C0002 X-Rspam-User: X-Stat-Signature: o8rqbnkmpzzdf736fis6zd8qkc3r55fe X-Rspamd-Server: rspam09 X-HE-Tag: 1755172719-925815 X-HE-Meta: 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 5AIx5kdP 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 14.08.25 13:38, Vernon Yang wrote: > Currently pagefaults on anonymous pages support mthp, and hardware > features (such as arm64 contpte) can be used to store multiple ptes in > one TLB entry, reducing the probability of TLB misses. However, when the > process is forked and the cow is triggered again, the above optimization > effect is lost, and only 4KB is requested once at a time. > > Therefore, make pagefault write-protect copy support mthp to maintain the > optimization effect of TLB and improve the efficiency of cow pagefault. > > vm-scalability usemem shows a great improvement, > test using: usemem -n 32 --prealloc --prefault 249062617 > (result unit is KB/s, bigger is better) > > | size | w/o patch | w/ patch | delta | > |-------------|-----------|-----------|---------| > | baseline 4K | 723041.63 | 717643.21 | -0.75% | > | mthp 16K | 732871.14 | 799513.18 | +9.09% | > | mthp 32K | 746060.91 | 836261.83 | +12.09% | > | mthp 64K | 747333.18 | 855570.43 | +14.48% | You're missing two of the most important metrics: COW latency and memory waste. Just imagine what happens if you have PMD-sized THP. I would suggest you explore why Redis used to recommend to disable THPs (hint: tail latency due to COW of way-too-large chunks before we do what we do today). So staring at usemem micro-benchmark results is a bit misleading. As discussed in the past, I would actually suggest to a) Let khugepaged deal with fixing this up later, keeping CoW path simpler and faster. b) If we really really have to do this during fault time, limit it to some order (might even be have to be configurable). I really think we should keep CoW latency low and instead let khugepaged fix that up later. (Nico is working on mTHP collapse support) [are you handling having a mixture of PageAnonExclusive within a folio properly? Only staring at R/O PTEs is usually insufficient to determine whether you can COW or whether you must reuse]. -- Cheers David / dhildenb