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Wed, 2 Sep 2026 06:09:00 -0400 (EDT) Date: Wed, 2 Sep 2026 11:08:59 +0100 From: Kiryl Shutsemau To: Pedro Demarchi Gomes 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=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam11 X-Rspam-User: X-Stat-Signature: f5pc6dnb6iyfp3sum86z6jkk66dtdht6 X-Rspamd-Queue-Id: 99C54A0009 X-HE-Tag: 1788343745-575496 X-HE-Meta: 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 sjynYXXO cVMJUzHMFxoz3MH07vlch+PUq7MpAHEXLNDjoeUBkPK4dWT9pC2VTuVhdry9+lDkZEsL/eyKN/qUUd8yL7FkjbrQrGU6kwIzyoeErrgEtKSzU3RW6Ne/aDaSYyWcG4XnYCJi8Rjr1bFuD2pfO0aU6lz7en1Ba0l0VwtwVhcsafSGTK867FrJx83gb8Qeuij8gVN9UsO2gbhE6ehZ53k0xzNeOER9zCwiXg1nLf1VL4jlQmRi7Olk6Y7jwswXNRsTRoaeCh8uQUYhwHKLY+l6GZzcp90RcJmONSvfqVQMJTrYk+oQXmXASwH+NaKhZm/s6rqwRD9Hdgi6+SOT25arOSfjPJ5WtXo5wzZ58E3XBzfiQztXPkuYVOdTXYSWQ4nkcVOd2PoolWcIFeRAyH+pmVkbC43naFqPo4gbg3T0VzFq3TyMvvIVMSoKmY+TCkJgj3nW3 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Aug 31, 2026 at 03:53:34PM -0300, Pedro Demarchi Gomes wrote: > 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. I am not sure it is worth as the moment. There's lower handing fruits in khugepaged that has to be addressed. -- Kiryl Shutsemau / Kirill A. Shutemov