From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9138B455630 for ; Sat, 19 Sep 2026 10:11:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789812667; cv=none; b=bgBiqN9KZWblghB2z6PhxlJoQipQJRcDExSnB0LuQPYO4wqXyl1YNLgfSvzituoQzw1mKBPOcoLuRV+qmy95VrMIBIyevFQdudy3T3VAkQ1hHx6wcGHKXU1gDtnHXI7gBmTSQS8ZzwbU1CDWK0Mwunql7/pyf4Ry9EaSh/6h1rY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789812667; c=relaxed/simple; bh=u6nMCYLRnxIInvBZMpdqR+U1ioovOqHCrQzmWVyEInY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=gN6VrI8iy77DKM7KF5KW7yJORDsA28rANnEMJxIANCyVdgZf7nlCxK1o9ewzs6naV46MKnLLitpBLgWfE86+lYDplKn4yEVoxARfKr0xMEkvf0Fv4wD4OhRHWw6pnwGLGG6wOzb8cluENeBPSJ6ju4YlXJaKO6DZxPAOj2I1Nyw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=l+5S651U; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="l+5S651U" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E73241F000FF; Sat, 19 Sep 2026 10:11:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789812666; bh=T6rLf9agDMyARwXvYrMSOSIy5jRBMFJCc5XZwhlIScw=; h=Date:Subject:To:Cc:References:From:In-Reply-To; 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 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