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b=M8vGXNiLZ4uoTTuW/1OzIrvqrP8aTwkJHYrLRW4RN0yvsEeEf4sa7nmL9znUdjY2h /COUlUvjP4OIf5nBqcmLqJbEGgEYxO6R61Bl8KJ9fE58urltoaB/ndm7+0FDrn7x/N oBp/wjqkhzAehzr9B18xzI6h2RY9Rzm3iUJuilnjGx/vsKVqAplg3SWuabgoQUuIYZ B83yy4zFM2SHk9Y0KESVhA5TvrgPwNi1b9Z2jwZlPE1RTo9uNxOGpYgEtIujH5AuSk vqubOBW0cweY0Aug77PlDNAesXkjZ9xXLD+AiexEmli11drGK9ZaQypvkdEnqvgxwk kN01hq29VEWqQ== Date: Tue, 7 Apr 2026 10:43:12 +0100 From: "Lorenzo Stoakes (Oracle)" To: Andrew Morton Cc: Hugh Dickins , xu.xin16@zte.com.cn, david@kernel.org, chengming.zhou@linux.dev, wang.yaxin@zte.com.cn, yang.yang29@zte.com.cn, Michel Lespinasse , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 2/2] ksm: Optimize rmap_walk_ksm by passing a suitable address range Message-ID: References: <20260212193045556CbzCX8p9gDu73tQ2nvHEI@zte.com.cn> <02e1b8df-d568-8cbb-b8f6-46d5476d9d75@google.com> <20260405140132.3a518740c6c1d68843f44aaf@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260405140132.3a518740c6c1d68843f44aaf@linux-foundation.org> X-Rspam-User: X-Rspamd-Queue-Id: 43976A0011 X-Stat-Signature: u4a97n67mi8dwb3ajdwze3a74a85ksxy X-Rspamd-Server: rspam06 X-HE-Tag: 1775554999-884580 X-HE-Meta: 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 jYGPO2rN ZggqS61ShpZCdidtqIaaLjNtlPuqGzicXAcCd6A4O5/y8GsbhfeQYPJ4UqFUz/+gJZLWZqpYyAnijD/GrO6DnP7hYrwCDZ5r3OgJvMEzbq5+MD7BrfeDo671xQllCO2bHKAr/Dlnh+lSltzA2UltpRtJvu93hjMjh3TEkMP6S2nAQYUnP6zyw/PYxz8IUW50AS+Nr0yn3bIvC2OTFUlqFU0kgjq8+9TrqS3avxPsx7eDT6FXvIo6PeLz0cOU2YgqRtm2mEXDqR+LkPb2h7QT6xsRWJpDJcvZdPeafpsBD621xZ9/FNIw3M2+pPa0L4sEeBeIR+n+pd8o+AiPBMIXEFjLCaKB0U7ZynShioO0RlOWI/JsKCkxrNNUFy+/xFGheYyG72ONcAkpJ3bA4ykwROxthaQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, Apr 05, 2026 at 02:01:32PM -0700, Andrew Morton wrote: > On Sat, 4 Apr 2026 21:44:14 -0700 (PDT) Hugh Dickins wrote: > > > This is a very attractive speedup, but I believe it's flawed: in the > > special case when a range has been mremap-moved, when its anon folio > > indexes and anon_vma pgoff correspond to the original user address, > > not to the current user address. > > > > In which case, rmap_walk_ksm() will be unable to find all the PTEs > > for that KSM folio, which will consequently be pinned in memory - > > unable to be reclaimed, unable to be migrated, unable to be hotremoved, > > until it's finally unmapped or KSM disabled. > > > > But it's years since I worked on KSM or on anon_vma, so I may be confused > > and my belief wrong. I have tried to test it, and my testcase did appear > > to show 7.0-rc6 successfully swapping out even mremap-moved KSM folios, > > but mm.git failing to do so. However, I say "appear to show" because I > > found swapping out any KSM pages harder than I'd been expecting: so have > > some doubts about my testing. Let me give more detail on that at the > > bottom of this mail: it's a tangent which had better not distract from > > your speedup. > > > > If I'm right that your patch is flawed, what to do? > > Thanks, Hugh. Administreevia: > > I've removed this patch from the mm-stable branch and I reworked its > [1/2] "ksm: initialize the addr only once in rmap_walk_ksm" to be > presented as a singleton patch. > > For now I've restaged this patch ("ksm: optimize rmap_walk_ksm by > passing a suitable address range") at the tail of the mm-unstable > branch and I'll enter wait-and-see mode. > Given we're at -rc7 now, I think we should delay this patch until 7.2, unless I'm much mistaken wrt Hugh's concerns. I'm concerned this is a subtle way of breaking things so we really want to be confident. We should also bundle up the test at https://lore.kernel.org/all/20260407140805858ViqJKFhfmYSfq0FynsaEY@zte.com.cn/ with this patch (should we find it's ok) as a separate series. Really overall I think safest to yank until 7.2 honestly. Thanks, Lorenzo