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Thu, 27 Aug 2026 10:07:10 -0400 (EDT) Date: Thu, 27 Aug 2026 15:07:10 +0100 From: Kiryl Shutsemau To: Lance Yang Cc: david@kernel.org, baolin.wang@linux.alibaba.com, pedrodemargomes@gmail.com, akpm@linux-foundation.org, ljs@kernel.org, ziy@nvidia.com, liam@infradead.org, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, usama.arif@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH mm-stable] mm/khugepaged: avoid unnecessary checking for swap entries when collapsing a mTHP Message-ID: References: <20260827121151.7837-1-lance.yang@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260827121151.7837-1-lance.yang@linux.dev> X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: D503D1C0017 X-Stat-Signature: aabhswrmsawiyij1pcwmo65m53xbcaue X-Rspam-User: X-HE-Tag: 1787839633-232392 X-HE-Meta: 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 MFb0Ixiz GwXSsSqz1XLgagpk33uVpP8BzcxhXb1jDCEVzsWc1NFVVHV87+j1U2ecd8Wf1oGUVjmxxDj3jouRYv/xFSzgyXaqPPsm04E+ZE6jRaBj5fAMkTA66ur/YgceLHtROs9UCzMTXUK40cXhCZBlaIJ6mm13m5OCx/zWjYO4TRyZeg26jKxH/xlT90JEpC2Kef6NbQ+iKBsNxzzGRISrH5472gpxud/99dMG7Cbs2otYcX87Tx9BDzPxgrfxnBogys8gbxTh39p4bxj/RqoF5ojSXCTBvNX++GeIxBJTGFgNhT6cjfJuvT7T/fS+E857twE7FL6HXhhdxWC9xnJfkjVaQsMfOLMqDYumAbZIOGuzAGcC1w3mdo/dB4CBEz8wQvHcefRx2EfMkt9uMFRpAtL1v65mQjyj+ZCj/+wXZWwB68zvO46MoSIMtv6q8EXtRDsG1rdiZ Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Aug 27, 2026 at 08:11:51PM +0800, Lance Yang wrote: > For the candidate above, the loop only reads its PTEs and returns > SCAN_SUCCEED. Pedro's bitmap lets mthp_collapse() determine that upfront > and skip the walk. > > Emm ... as I asked before[1], any numbers showing how much the extra scan > costs? With the win being only some scan cost, I don't think it is going to be measurable. It can open a path to future work. For example, we could find fully populated mTHP windows in a PMD that is not eligible for collapse because unmapped > max_ptes_swap. Right now we just give up on the whole PMD in such cases. But there are more wins to gain there. Just saying :P -- Kiryl Shutsemau / Kirill A. Shutemov