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Mon, 31 Aug 2026 11:55:26 -0700 (PDT) Date: Mon, 31 Aug 2026 15:53:34 -0300 From: Pedro Demarchi Gomes To: Kiryl Shutsemau Cc: Lance Yang , david@kernel.org, baolin.wang@linux.alibaba.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=utf-8 Content-Disposition: inline In-Reply-To: X-Stat-Signature: iwf91qw7eeyb3zurjb6xbu31styidpor X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: E20B120002 X-Rspam-User: X-HE-Tag: 1788202527-34517 X-HE-Meta: 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 jwllDd/A b0+nOILEIrTFY4WMkCBWszuBo+zcukaHHX9g2ot9w9hUN4Q7WR0E94oNtbNhdY2hIsJs/+ulcxBQPDLUAQPivrVT5rHecV1RjQyDiawgyNVvLalSjop1wh7LHojJ9bJ4N+LsyTHxLth1zD8hYYs4MojvleTDkfKRFuvRSjCa1O8ERf9wqqmmimCCAgmVIAIpuCDOxmarLQVhYGeKH0uIKjsTg+0na6kApjsXD8GDJHZgSCKDKSCPJUBfzL62oSSnC1+UuN9xGFhxlrwLKgjR2u5qs8VPahaH+4hjf/bC7dXpQjtFvBLcU8BMm6b1Y0sZel9l8c6k35MFWKo6RbWQWV8ZZKpaeQZVAOIdD+MEjgkdq1lsH7uAAtd6Q5mcDa4znGhQHWEF4OKJl1PgoifsyNnTWVFoQvsAi3uQGdGB4ZBpAFpR7mt1/EJ/HY/q2cl7EJgerpzoSMSXj4AH+CCOP3yXV2m/YhoKEGoYM 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 03:07:10PM +0100, Kiryl Shutsemau wrote: > 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 Hi Kiryl, I devised a patch based on your proposed future work idea. If you think it makes sense, I can send it to the mailing list with a Suggested-by: tag. Thanks!