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b=XBKD6Gfd5BTPGg8/bdEfNKvUOJkpXmg9TRc6RFjcCMacqkeMHrkrJrC3wlPDm2uJy SZMe6dox0ys34wBPeN6gaXqMPzEYYl5JpMCOWQOISRJBi0GUh+dSok7nnz4WKSJbzi Vm4uWYL5K/qiGniHPJ7j3KdCToV6ksgBKNz1htLY= Date: Sun, 3 May 2026 07:59:44 -0700 From: Andrew Morton To: Cc: , , , , , Subject: Re: [PATCH v4 0/5] KSM: Optimizations for rmap_walk_ksm Message-Id: <20260503075944.0c19e048e7e20f6715d9e86e@linux-foundation.org> In-Reply-To: <20260503203538194jFwVGloy43M1F3sQGaFt7@zte.com.cn> References: <20260503203538194jFwVGloy43M1F3sQGaFt7@zte.com.cn> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Stat-Signature: oyejcghwqoei1sqi88bgw5nhochqwd58 X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: CC0A64000A X-Rspam-User: X-HE-Tag: 1777820390-429091 X-HE-Meta: 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 fC3u+Frc g5HjC0Rx+oCXY+JaaExmBCpWfAR9X+sUPNB15rj7M2QHwIEokQStyUIgphmf76Rq2EiIjrEafldc7LPQtYOIPvzoues9swWfPMV8nURgZApDKHhRQPUGn0iSWRIduZ3sYZMFKelqXGYrmbmhpkcgzSn2swhGU7XoGKH6ctWbaSnSm25Gg1QE8Ekmi4iXw/hAIWJPUISrOja3AYO4GQTh+O7EvXK+NVI3bMvgy2Cy3Q7DhCFiE9DvjqbR1MhztfPukoqBaMCUPXCPOnXHunsRV9xhE37tdfpbjQDGa0rKUGrD/++axeXJ7ffdk2lnybQFsvC/GGdqIl8hfkMvsPs632NYGXn5GoDHbACMWRrYgdyoXuryeTJe3bAugQHlYBiymHGlEYQ+M+35lUD+aVQ5NPnlBULOSbCRq47QtQL279xTVpKJUk0iuxbrHPNKwb/+P72UooBjt6VmzEia4MjyDV6EW/IO/330/CqTL+atjxYOTOLI= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, 3 May 2026 20:35:38 +0800 (CST) wrote: > Deep investigation revealed that rmap_walk_ksm's 99.9% of iterations inside > the anon_vma_interval_tree_foreach loop are skipped due to the first check > "if (addr < vma->vm_start || addr >= vma->vm_end)), indicating that a large > number of loop iterations are ineffective. This inefficiency arises because > the pgoff_start and pgoff_end parameters passed to > anon_vma_interval_tree_foreach span the entire address space from 0 to > ULONG_MAX, resulting in very poor loop efficiency. > > An initial immature thought was using the "rmap_item->address >> PAGE_SHIFT" > to be the pgoff passed into anon_vma_interval_tree_foreach(). But this is > flawed because 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, which was pointed out at: > > https://lore.kernel.org/all/02e1b8df-d568-8cbb-b8f6-46d5476d9d75@google.com/ > > According to the implementation of anon_vma_interval_tree_foreach — > it essentially iterates to find a suitable VMA such that the provided pgoff falls > within the VMA's range [vm_pgoff, vm_pgoff + vma_pages(v) - 1]. > > So the solution is to add vm_pgoff field in ksm_rmap_item and use vm_pgoff instead of > address >> PAGE_SHIFT. Thanks for pushing ahead with this. Regarding the [4/5] changelog: I don't think I understand how much effect this change has upon real-world workloads. Are you able to clarify that? "How useful is this change to Linux users". AI review had a lot to say: https://sashiko.dev/#/patchset/20260503203538194jFwVGloy43M1F3sQGaFt7@zte.com.cn Human review was wondering how much overhead [1/5] would add. I do note that it adds overhead even when CONFIG_TRACING=n - the rmap.o text segment gets a few hundred bytes larger and there's additional runtime overhead.