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[2003:d8:2f23:c700:d8ae:41bf:492a:9e4c]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45a22210ab4sm44725805e9.3.2025.08.15.23.40.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 15 Aug 2025 23:40:38 -0700 (PDT) Message-ID: Date: Sat, 16 Aug 2025 08:40:37 +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 Cc: 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, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250814113813.4533-1-vernon2gm@gmail.com> <20250814113813.4533-7-vernon2gm@gmail.com> <226e21a5-8a8e-43a0-bf5f-12a761f49a27@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: q4epbrxsxfxezpye94mnji4u77e95yia X-Rspam-User: X-Rspamd-Queue-Id: 02CDDC000A X-Rspamd-Server: rspam01 X-HE-Tag: 1755326443-367576 X-HE-Meta: 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 OufGzV3M 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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 15.08.25 17:20, Vernon Yang wrote: > On Thu, Aug 14, 2025 at 01:58:34PM +0200, David Hildenbrand wrote: >> 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. > > OK, I will add the above two test later. > >> >> 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). > > Thanks for the suggestion, I'm not very familiar with Redis indeed. Currently, > this series supports small granularity sizes, such as 16KB, and I will also > test redis-benchmark later to see the severity of tail latency. > >> >> 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). > > This is a good way to add a similar shmem_enabled knob after if need. > >> >> 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]. > > There is no extra processing on PageAnonExclusive here, only judging by R/O PTEs, > thank you for pointing it out, and I will look into how to properly handle > this situation later. Yes, but as I said: I much prefer to let khugepaged handle that. I am not convinced the complexity here is warranted. Nico's patches should soon be in shape to collapse mthp. (see the list) -- Cheers David / dhildenb