From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A87612EEE76; Thu, 11 Jun 2026 16:37:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781195843; cv=none; b=dHP0g8bE8B3wQ1iFmWxnFxZTD4JqbR8vJePfrBWlLOrf3OSN6AZ6rsZt7kF60+tm3pJm6RLQyRY8LKUL4LMixX/G5kjIQmq2a61w5u6WX4hAEVWfCU9rLtfvLW9gJ0BYkpqhUxwet8/lBVyQyeSa6m11624TQUIiBIXLjPWMQn0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781195843; c=relaxed/simple; bh=9xDkjo5I0dtJERSCveIFJwIPv1OZLdEPoSQrZKkXzeo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=G0bhFg4nMy4rQvOslimswx3qO/C9OVQLkFiP1yd21Y414PYI+U3aeI1XIPfyyNkrvqtp0ziiVpQeQ1U/EiwfngWgCiv3o70LoZWanNuEGNUImJI/kvwlhoN19tFfxp5W91faAOXeB5hssxTVALt+sPHyR7MPjCVDMxEBs7uaxlE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=YPqKVI7T; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="YPqKVI7T" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 68C081F00893; Thu, 11 Jun 2026 16:37:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781195842; bh=KQBib1VsAzlYorWOyjFbpOR4CJ/YC0cjrLSbb74DjF0=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=YPqKVI7T54I7X0z7db2P0pkwgtlgSotue8EuNyxIeEdfPbnmla2HXzb9/l6g8uTKI A+BUzA5ep4mCpPRN3mCkJyae3Dr4ZalAPFtE9g91OpmYbCQrioVqHkDSf7hAOklG/2 /WgJmupLkZRfQRuNua4Mohi6MkfVQu4f8A/gtWLiPxOGGTJjwZFlCy0w2Vdk0b8PAG bDJWYiiRne0yFSxex7rZ9fIoCauSch327g7pm4GCcjaATjbgHH+ShhHfX8ywBG8vZs 5C9S/ZSrld0RQWr2R8ECgNOf3Duz1FUwbMmZ+++Bggc3Vh+sT6krASx8cptrrl6BMD f2jkavqmju5cA== Message-ID: <4cf98483-ae35-4ad0-8f77-5a46194eb65f@kernel.org> Date: Thu, 11 Jun 2026 18:37:16 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 02/16] mm/slab: do not init any kfence objects on allocation Content-Language: en-US To: Harry Yoo Cc: Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Suren Baghdasaryan , Alexei Starovoitov , Andrew Morton , Johannes Weiner , Michal Hocko , Shakeel Butt , Alexander Potapenko , Andrey Konovalov , Marco Elver , Dmitry Vyukov , kasan-dev@googlegroups.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org References: <20260610-slab_alloc_flags-v2-0-7190909db118@kernel.org> <20260610-slab_alloc_flags-v2-2-7190909db118@kernel.org> <159d1e20-5b21-4329-ac9a-f7a5cb0fd56a@kernel.org> <74adf668-78c2-4989-a6c6-c6ec7bd68855@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/11/26 17:11, Harry Yoo wrote: > >> From 3a1c4398ce9f361a4e6f4d9946eab6237eea89c2 Mon Sep 17 00:00:00 2001 >> From: "Vlastimil Babka (SUSE)" >> Date: Wed, 10 Jun 2026 17:40:04 +0200 >> Subject: [PATCH] mm/slab: do not init any kfence objects on allocation >> >> 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 perfoms the zeroing on > > nit: perfoms -> performs Fixed. >> its own because 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. > > Right. > >> Thanks to Harry Yoo for the initial refactoring attempt, and for updated >> comments that are used here. > > No problem ;) > >> Link: https://patch.msgid.link/20260610-slab_alloc_flags-v2-2-7190909db118@kernel.org >> Signed-off-by: Vlastimil Babka (SUSE) >> --- > > Looks good to me, > Reviewed-by: Harry Yoo (Oracle) Thanks! > Thanks! >