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Wed, 31 Dec 2025 04:13:16 -0800 (PST) Received: from localhost.localdomain ([121.232.80.251]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2a2f3d4cbcfsm331485985ad.50.2025.12.31.04.13.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 31 Dec 2025 04:13:15 -0800 (PST) Date: Wed, 31 Dec 2025 20:13:08 +0800 From: Vernon Yang To: "David Hildenbrand (Red Hat)" Cc: akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, richard.weiyang@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Vernon Yang Subject: Re: [PATCH v2 3/4] mm: khugepaged: set VM_NOHUGEPAGE flag when MADV_COLD/MADV_FREE Message-ID: <4y4aht35lkswkaorr36m6276aktp65bywdtnj6sxo7koscj3qp@qpdjv47lc75v> References: <20251229055151.54887-1-yanglincheng@kylinos.cn> <20251229055151.54887-4-yanglincheng@kylinos.cn> <084eee6b-6c9e-454b-a563-b2babb76b099@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <084eee6b-6c9e-454b-a563-b2babb76b099@kernel.org> X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 4DC3A8000D X-Stat-Signature: 6aug76pbtc7azq61r9eo3ne49dx1xoya X-Rspam-User: X-HE-Tag: 1767183197-706507 X-HE-Meta: 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 fu2c/7Ca 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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 Tue, Dec 30, 2025 at 08:54:33PM +0100, David Hildenbrand (Red Hat) wrote: > On 12/29/25 06:51, Vernon Yang wrote: > > For example, create three task: hot1 -> cold -> hot2. After all three > > task are created, each allocate memory 128MB. the hot1/hot2 task > > continuously access 128 MB memory, while the cold task only accesses > > its memory briefly andthen call madvise(MADV_COLD). However, khugepaged > > still prioritizes scanning the cold task and only scans the hot2 task > > after completing the scan of the cold task. > > > > So if the user has explicitly informed us via MADV_COLD/FREE that this > > memory is cold or will be freed, it is appropriate for khugepaged to > > skip it only, thereby avoiding unnecessary scan and collapse operations > > to reducing CPU wastage. > > > > Here are the performance test results: > > (Throughput bigger is better, other smaller is better) > > > > Testing on x86_64 machine: > > > > | task hot2 | without patch | with patch | delta | > > |---------------------|---------------|---------------|---------| > > | total accesses time | 3.14 sec | 2.93 sec | -6.69% | > > | cycles per access | 4.96 | 2.21 | -55.44% | > > | Throughput | 104.38 M/sec | 111.89 M/sec | +7.19% | > > | dTLB-load-misses | 284814532 | 69597236 | -75.56% | > > > > Testing on qemu-system-x86_64 -enable-kvm: > > > > | task hot2 | without patch | with patch | delta | > > |---------------------|---------------|---------------|---------| > > | total accesses time | 3.35 sec | 2.96 sec | -11.64% | > > | cycles per access | 7.29 | 2.07 | -71.60% | > > | Throughput | 97.67 M/sec | 110.77 M/sec | +13.41% | > > | dTLB-load-misses | 241600871 | 3216108 | -98.67% | > > > > Signed-off-by: Vernon Yang > > --- > > As raised in v1, this is not the way to go. Just because something was once > indicated to be cold does not meant that it will stay like that forever. > > Also, > > (1) You are turning this into an operation that will perform VMA > modifications and require the mmap lock in write mode, bad. > > (2) You might now create many VMAs, possibly breaking user space, bad. > > If user space knows that memory will stay cold, it can use madvise() to > indicate that these regions are not a good fit for THPs. > > But are they really not a good fit? What about smaller-order THPs? > > Nobody knows, but changing the behavior like you suggest is definetly bad. > :) > Thank you for review and explanation. I got it. For MADV_FREE, we will skip the lazy-free folios instead. For MADV_COLD, it will be removed in the next version. -- Thanks, Vernon