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From: Shakeel Butt To: "Vlastimil Babka (SUSE)" Cc: Harry Yoo , Andrew Morton , Roman Gushchin , Hao Li , Christoph Lameter , David Rientjes , Suren Baghdasaryan , Usama Arif , Meta kernel team , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Danielle Costantino Subject: Re: [PATCH] mm/slub: serve slabobj_ext array from a strictly larger kmalloc cache Message-ID: References: <20260625230029.703750-1-shakeel.butt@linux.dev> <62453403-954c-4cf1-8924-6d38184b0810@kernel.org> <09267187-6c85-438f-8791-4cce8d07892a@kernel.org> 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=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT On Sat, Jun 27, 2026 at 07:58:12PM -0700, Shakeel Butt wrote: > On Fri, Jun 26, 2026 at 07:11:33PM +0200, Vlastimil Babka (SUSE) wrote: > [...] > > >> > Fix it structurally by removing cycles of every shape: serve the array > > >> > from a cache strictly larger than the one it describes whenever it would > > >> > otherwise come from the same or a smaller cache. Every reference edge > > >> > then points from a smaller to a larger cache (here kmalloc-1k's array > > >> > moves to kmalloc-2k), so the relation is a DAG and cannot contain a cycle. > > >> > > >> This will fix the problem. > > >> > > >> But this will waste memory as we need smaller obj_exts array > > >> as the size gets larger. > > >> > > >> We should probably create a new kmalloc type to avoid cycles instead? > > >> (needed only when memory profiling is enabled, though) > > >> > > >> That would also prevent recursion even further. > > > > > > Yes but I assume that would add kmem caches even for users not using memory > > > profiling. Anyways, I think that is a separate discussion. Am I understanding > > > correctly that you don't have any concerns with this approach? > > > > Umm, the memory waste is a concern? > > > > Minimally I'd now want to only do that size bumping when allocation > > profiling is enabled. Ideally that means both configured in and not booted > > with "never". > > > > We probably should have done that already in 280ea9c3154b2. Because AFAIU > > memcg-only obj_exts array don't have this issue (or maybe they do have the > > [1] issue? Harry?). But if memcg-only should keep avoiding the same size > > bucket, it can keep what it was doing and only memalloc profiling would do > > the strictly larger thing. > > memcg should not have this issue as normal kmalloc caches do not serve memcg > charged objects. I am wrong here as I went back and see d8df600b67d7. > > So here we can do dedicated caches as Harry suggested or make this size bumping > very specialized as Vlastimil suggested. What do we want long term? Orthogonally > we do want this fix to be backported easily to older stable kernels. I will see > how does this narrowed down size bumping looks like. > BTW I think we need something like the following, right? if (mem_alloc_profiling_enabled()) { if (obj_exts_cache->object_size <= s->object_size) return s->object_size + 1; } else { if (obj_exts_cache->object_size == s->object_size) return s->object_size + 1; } > > > > Suren's input would be also nice to have. > > > > Thanks! > > > > [1] https://lore.kernel.org/oe-lkp/202601231457.f7b31e09-lkp@intel.com > >