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From: Hao Li To: "Vlastimil Babka (SUSE)" Cc: Harry Yoo , Christoph Lameter , David Rientjes , Roman Gushchin , Suren Baghdasaryan , Alexei Starovoitov , Andrew Morton , Johannes Weiner , Michal Hocko , Shakeel Butt , Alexander Potapenko , Marco Elver , Dmitry Vyukov , kasan-dev@googlegroups.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org Subject: Re: [PATCH v3 01/15] mm/slab: do not init any kfence objects on allocation Message-ID: References: <20260615-slab_alloc_flags-v3-0-ce1146d140fb@kernel.org> <20260615-slab_alloc_flags-v3-1-ce1146d140fb@kernel.org> Precedence: bulk X-Mailing-List: cgroups@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: <20260615-slab_alloc_flags-v3-1-ce1146d140fb@kernel.org> X-Migadu-Flow: FLOW_OUT On Mon, Jun 15, 2026 at 01:54:34PM +0200, Vlastimil Babka (SUSE) wrote: > When init (zeroing) on allocation is requested, for kmalloc() we > generally have to zero the full object size even if a smaller size is > requested, in order to provide krealloc()'s __GFP_ZERO guarantees. > > When we end up allocating a kfence object, kfence performs the zeroing > on its own because it has its own redzone beyond the requested size. > Thus slab_post_alloc_hook() has an 'init' parameter which has to be > evaluated in all callers (via slab_want_init_on_alloc()) and should be > false for kfence allocations. > > For kfence allocations in slab_alloc_node() this is achieved by subtly > skipping over the slab_want_init_on_alloc() call. Other callers (i.e. > kmem_cache_alloc_bulk_noprof()) however evaluate it unconditionally even > if they do end up with a kfence allocation. This is only subtly not a > problem, as those are not kmalloc allocations and thus the "requested > size" equals s->object_size and thus it cannot interfere with kfence's > redzone. There's just a unnecessary double zeroing (in both kfence and > slab_post_alloc_hook()), but it's all very fragile and contradicts the > comment in kfence_guarded_alloc(). > > Remove this subtlety and simplify the code by eliminating the init > parameter from slab_post_alloc_hook() and make it call > slab_want_init_on_alloc() itself. Instead add a is_kfence_address() > check before performing the memset, which will start doing the right > thing for all callers of slab_post_alloc_hook(). > > This potentially adds overhead of the is_kfence_address() check to > allocation hotpath, but that one is designed to be as small as possible, > and it's only evaluated if zeroing is about to happen. This means (aside > from init_on_alloc hardening) only for __GFP_ZERO allocations, and the > zeroing itself comes with an overhead likely larger than the added > check. > > While at it, refactor the handling of evaluating when KASAN does the > init instead of SLUB, with no intended functional changes. A > non-functional change is that we don't pass kasan_init as true to > kasan_slab_alloc() if kasan has no integrated init, but then the value > is ignored anyway, so it's theoretically more correct. > > Thanks to Harry Yoo for the initial refactoring attempt, and for updated > comments that are used here. > > Link: https://patch.msgid.link/20260610-slab_alloc_flags-v2-2-7190909db118@kernel.org > Reviewed-by: Harry Yoo (Oracle) > Reviewed-by: Suren Baghdasaryan > Signed-off-by: Vlastimil Babka (SUSE) Reviewed-by: Hao Li -- Thanks, Hao