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b=l+5S651UOr8S1vO1bGledRyKS7GVLLbQpEy8lMv6ldvYtkMWP5Ze0XUgkcqYdSyCR 0ya401ohT1jtFdfAvPQjqs7+2oF71svgvlRCBIXRlHNTFcNSfPMtp98fR4uWffdHTY xDGGk5PyVUHwY2QOzMfqrj/DTJJR9mfmwTS0V1lB+DGEBUgureD9MmCdThXorMwTfC uRcldTNp9FbPeG+pG/oy+L/gJsuoFJ49auy+m4Kw0hz/VMTxpWTb9holgN+K3bjCDD fsG3oPLdi4iKvouNiOD4PvbeM1BPduGrcO9rIQgox6vSRBG30hqnXx2XiUqPSk3xVj ElPtZvU5oLD1Q== Message-ID: <0c0c19d0-997a-4c8d-a103-2b3d153f4e32@kernel.org> Date: Sat, 19 Sep 2026 12:10:56 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/2] mm/memory: reuse the whole exclusive large folio on a write fault To: Yuan-Hao Hsu , Andrew Morton Cc: Lorenzo Stoakes , liam@infradead.org, Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Barry Song , Ryan Roberts , Dev Jain , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260918064238.868-1-aa9736195201@gmail.com> <20260919073134.639-1-aa9736195201@gmail.com> <20260919073134.639-3-aa9736195201@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam04 X-Rspam-User: X-Stat-Signature: 7eu9efz3ygq579zqx3soykd1p7wh7ikr X-Rspamd-Queue-Id: 96FDB80006 X-HE-Tag: 1789812667-562777 X-HE-Meta: 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 I4LB3P4+ LhqGYHstxpuXpSsJ2xhedTPODIuzn9befSkgguaVWbW2anWkBnz0Hrm2134CsnIkBACOqgacqwdQpKFIYF+9Mt6jKzJEmPvHcyXhmlPWiXeapNblJzregN/y2oI8ThD8LWrQCl1STPn2m0RiKHlsWETj/jRiVX/OSfjPTNEYOWyXoJ05fVcHkmtIIu2pA82gSgluFFXPXCIIM95TjRhoHEM7pzG7UXNuI61JNz4s8lhUpPnEAB36lUkEuI5kfXISdRlT9 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/19/26 09:31, Yuan-Hao Hsu wrote: > With blocks of 16 PTEs a PTE-mapped 2M THP still takes 32 reuse faults > and a 1M folio 16. Lift the bound to the page table: the walk then > covers the PTEs that map the folio in this page table, at most > PTRS_PER_PTE of them, and one fault does the work for the folio. > > Its cost, measured as the time of the store that takes it, against > 420 ns for a reuse fault today (x86-64, i7-12700KF, medians, ns): > > reuse fault fault that COW fault that > (patched) allocated it copies 4K > 1M mTHP 5,400- 5,700 63,000-68,000 1,500 > 2M THP, PTE-mapped 10,000-10,500 126,000 1,500 > > That is 14-20 ns per PTE, about 2 ns of it the scan. Builds that > differ only by NOPs in front of the function take either 10,100 or > 7,500 ns for the 2M folio, with a period of 32 bytes: the loop of > modify_prot_commit_ptes() changes speed with its address. > > What it buys, 256 MiB after fork() and the child's exit, medians of 15 > runs, two boots of each kernel: > > 16 PTEs whole folio > one byte per page, seq > 1M mTHP 5.7 / 5.8 ms 4.1 / 4.0 ms > 2M THP, PTE-mapped 5.7 / 5.8 ms 3.9 / 3.9 ms > one byte per page, random order > 2M THP, PTE-mapped 7.4 / 7.5 ms 4.9 / 5.1 ms > memset() > 2M THP, PTE-mapped 40.2 / 39.4 ms 38.1 / 37.2 ms > 8 threads, random order > 2M THP, PTE-mapped 0.9 / 1.0 ms 0.7 / 0.7 ms > one store per 64K, 2M folios 3.3 / 3.3 ms 1.5 / 1.6 ms > one store per 2M, 2M folios 0.1 / 0.1 ms 1.4 / 1.4 ms > Are you measuring the worst-case latency of a single write fault or only the average over all writes? -- Cheers, David