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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> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: 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!