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c=simple/simple; d=arm.com; s=foss; t=1794595657; bh=FlyuBewiCDaPu1+s79sWx/lfGstyIBmDKHKwd7lp/vM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=bieAlkYBHpiq7QexIl6ZIv6M7QaRTP5zcRITuv0go21Gf47D3IhN291ITaYxrPNPD WP+N1VV8XUBe2tMhPl8J4zlu4fJoNSx3g2TVg15aXVlmGGEXdFO5JvV6Sz2g5qC0wZ dide5nesRF9/b94f0Vka18t5XTni0ynuw8xegKs8= Date: Fri, 13 Nov 2026 18:47:32 +0000 From: Catalin Marinas To: Breno Leitao Cc: Vlastimil Babka , Harry Yoo , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH v2] slab: allow sheaves for SLAB_NOLEAKTRACE caches Message-ID: References: <20261113-kmemleak_sheaves-v2-1-8c557c66cec2@debian.org> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261113-kmemleak_sheaves-v2-1-8c557c66cec2@debian.org> X-Stat-Signature: ds6wro7tn915c8tckqsmx997noqjsr5a X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 0474D160004 X-Spam-Status: No, score=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-HE-Tag: 1794595658-480709 X-HE-Meta: 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 QZ2AimvX xJ1KkD5iVs1LAVCnwwAM5HIAmvSnI6TM6a4ViEzi0lVgaVrPtrwWfAxLnL10T1J/iubdtoPByNhEkVzlvgP1e6df6+dnxZdANE6V10Fym5b+050GI3D9tCnNG2LX5+jUR5uwZCHpVB1xq8Jy6MhRtiFlODFbReQtD5rGg1haUGqt073WO346Yq27rs+VMst2lufKyCMoB2G1y2LeYFCl+rVoRDkRmm3IJJGrhBNLL0Ea0xcPiVQq3nZyet/LnRBszDjLvFtqM+LnFmBiRtIO1cf5Hiz8d43ZedWxXVn9BeNCECtGiJBs9FxiSiqishc2ImcL3FfTIW8CVSREHCTEiBZmLqGle5Kzw9yX775bSxtL80pvg4ohdrmFN2Il35pTTIAXNb3Ac4JZPsZYb4Rfpf3GHm8GZTevl2MGx4uhitceVLFvLRyBgrw2CH/RFxzgL+RxwhNzKYd4Ks9zozfjSVaJUw6PypA7kxGDOC+VngdYz+fY= 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 Fri, Nov 13, 2026 at 06:36:56AM -0800, Breno Leitao wrote: > This is a patch that makes kmemleak cheaper to run, by letting > kmemleak's own metadata cache use a _new_ sheaves type. The flag only > makes that possible without recursion. > > The cost today: > * every allocation kmemleak tracks needs one kmemleak_object from > object_cache. An RCU callback frees it later. > > Why that's slow: > * object_cache has no sheaves, so each of those allocations and frees > goes to the shared slab lists under n->list_lock. > - On meta's fleet kmemleak kernels, list_lock is the second most > contended lock, after kmemleak_lock. > - About 88% of that wait is RCU callbacks freeing kmemleak objects. > > Why sheaves aren't allowed: > * a sheaf is a kmalloc object. Tracking it makes kmemleak allocate > another kmemleak_object, which needs a sheaf, and so on. > > When I reported the recursion in [1], I suggested keeping > SLAB_NOLEAKTRACE caches away from sheaves, and commit e47c897a2949 > ("slab: add sheaves to most caches") did that. It stopped the > recursion, but it took the sheaves away from the whole cache, although > only the kmemleak tracking of the sheaf itself recursed. > > Commit 17c38c88294d ("slab: remove cpu (partial) slabs usage from > allocation paths"), from the same series, then left these caches with > no per-CPU caching at all. > > In hindsight, skipping kmemleak only for the > sheaf itself, as the new SLAB_ALLOC_NO_LEAKTRACE does, would have been > the better fix. > > The sheaf is allocated with SLAB_ALLOC_NO_RECURSE as well, like kmalloc > sheaves. Without it, a backing kmalloc cache that needs a new sheaf of > its own allocates it with SLAB_ALLOC_NO_LEAKTRACE too. kmemleak then > does not track that sheaf, and can report objects queued in it by > kfree_rcu() as leaks. > > SLAB_ALLOC_NO_RECURSE also skips allocation tagging, so mark the codetag > of these sheaves empty on free, as is done for kmalloc sheaves. > > Link: https://lore.kernel.org/all/aW_NK2NXVgtuzCVH@gmail.com/ [1] > Cc: Catalin Marinas > Reviewed-by: Catalin Marinas > Signed-off-by: Breno Leitao > --- > Changes in v2: > - Fix sashiko findings: > - Set SLAB_ALLOC_NO_RECURSE along with SLAB_ALLOC_NO_LEAKTRACE, so the > flag stays off the kmalloc cache's own sheaf allocation, per Catalin > Marinas. > - Mark the codetag of SLAB_NOLEAKTRACE sheaves empty on free, per > Catalin Marinas. > - Link to v1: https://patch.msgid.link/20261112-kmemleak_sheaves-v1-1-e40e3d73b253@debian.org v2 looks fine, so my reviewed-by stands. Thanks for respinning. -- Catalin