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Thu, 22 Oct 2026 07:29:18 -0700 (PDT) Date: Thu, 22 Oct 2026 15:29:18 +0100 From: Matt Fleming To: Marco Elver Cc: Caleb Kan , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com, Vlastimil Babka , Alexander Potapenko , Dmitry Vyukov , Andrey Konovalov , Oscar Salvador , Caleb Kan , kernel-team@cloudflare.com Subject: Re: [PATCH RFC v2 00/11] stackdepot: reduce memory use for persistent stack records with a trie Message-ID: References: <20260908-stackdepot-trie-v2-0-1996d5cef732@cloudflare.com> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: C42D44000E X-Rspam-User: X-Stat-Signature: 8recm5349zxfgb8ssqas3wknodb971hy X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS autolearn=no autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam03 X-HE-Tag: 1792679360-741975 X-HE-Meta: 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 c2KMXEwV 7F+q0+1QlfcIpamlzADhj2NNiiPm3MDT1DqQm46jEPnoMRe/EHm6lBjDb6fe+ldhRBSLITkV+Qk82FLkcZoMz0hKsyLe6k3aKnA1BxKuHgWGNwAyi5mhuzQm+q3xnBo2oPp1GzwcK7q0wgyR5Hp00t7onijbhAwmbE7BeHTaGCRQr5KFPNwBzNkietB7Q915DQOPi0fpUhmuZNUs4DU0zRKJWM3p6Q7SX3GEAm0zJdEUT2hQuTf3jPMMcCVGFTkDNsaaFBqCzIrm0yeDmVuGAaOdoKQvCVXue1ZDHmq9Z3edciqMthfOPfqaITPXYHRnmQkxRO81wlU4wG9DrS0vJhOKmhoA/O5y0+KW2ptHtmqvwYFSI82iHsw6hA9qArd58dlBOIZoJ59+zaGtjihn9FExIu1Jy0Ia815g0SqJDIz1hnGzdBI1lBsu3Zp6Q2av6bu3Eni0Xr5iLShmjfHARhhRkRg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Tue, Oct 20, 2026 at 03:07:40PM +0000, 'Marco Elver' via kernel-team wrote: > On Tue, Sep 08, 2026 at 02:13PM +0100, Caleb Kan wrote: > > Hi, > > > > This series reduces the memory used by persistent stack depot records by > > sharing common frame prefixes in a path-compressed trie. > > > > Cloudflare runs KASAN on pre-production servers so allocation and free > > stack traces are available when diagnosing memory-safety bugs. On some of > > these servers, stack depot exhausted its pool budget even after we raised > > stack_depot_max_pools from 8,192 to 32,768. Once the depot is full, a new > > persistent trace returns no handle and a later KASAN report can lose the > > history needed to explain the bug. > > > > With 4 KiB pages, 32,768 order-2 pools allow 512 MiB of stack storage. > > Doubling the limit again would allow 1 GiB, but it would not change the > > linear growth: the hash backend shares identical complete traces, while > > two traces that differ by one frame are still stored independently. > > > > The trie is opt-in and disabled by default. > > Have you measured how much of the memory reduction comes simply from > storing 32-bit compressed frames (patch 9) directly inside the existing > hash-backed struct stack_record? That alone cuts frame storage by > ~40-45% across *all* callers (including page_owner and KASAN aux stacks) > with O(1) lookups and none of the trie/bitmap/RCU complexity. If prefix > sharing is still needed on top of that, deduplicating fixed-size (e.g. > 4- or 8-frame) immutable segments from caller to callee in the existing > hash table (chained via parent_handle) would avoid mutable child arrays, > RCU retirement, and the bitmap allocator entirely. No, I don't think we've done that. But I will run the numbers and report back. Thanks, Matt