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 DF4613DB300; Wed, 17 Jun 2026 14:37:04 +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=1781707026; cv=none; b=ciasqjyXvbS+r9PunThhrgo8kcg2z0g2jV47Sv8pceIQt/oZ5rBpGhUB5P5zhXkdOvGCxSfMqyNJylXydTqOM/L4ScnQ/WstrB+t7vof0bfE7sTgz+/4dcPVo49M2SvTedemj/9NKrrzA7t2JsEdU0COP7nIi/Wf8yVRwaGkgJY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781707026; c=relaxed/simple; bh=q0XAVA1HHCpriLNnEZ5hJgBPnz8e6cCzLZ2diM9TKZg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=QnuAKrdpjAY5jpDNpWzeS3xKulFInnXw2IxLD24k7ThgbHwi+LQMtc79ncPULEbuVsULmJ+g2XodcQNCjGwbavs4nqIhkBULyGjbBM8rx5mG8FvCxkBv4AhKka+OyyrRYfq8YkZOwGIMeC0u5C1D/rMw0LuJcdlppPL1bMP6wEk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=IcHt3Y0P; 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="IcHt3Y0P" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2FA2B1F00A3A; Wed, 17 Jun 2026 14:37:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781707024; bh=1K7NdwWrShKdIgD+8OvjC8OfnEWC3QHLcX5exYj1THI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=IcHt3Y0PPyWEmxddWmKgfJU/e4Yw/MO27OJ7f4o1fS1EolxhseUUSqYF8iWgMlOdl SLVbKiYVS9Duyfw0e1s1Q7lwrJ98twNMQKNB9PfA5rzqOLG+XgCJGlezC2z2H88MG6 P9apbRVSKmAJ+bx/kYzGpBYNaLae3XeeQv6s4XHbPGDK31wkUep/7n2SdsPHjZeJ7g mBQ6bDPcTBJrj61zubrd4G+KXjFcZDfwcwYVbdrQWBgyYTfqO26e9YKzI4w1isGdOg WPBLE8bM1RoD3g05soaZHOdU/KViE3adlqRjC+MLaOjH/H0AWDtEBjq1BHjsQzr65M KyOPKxc05SWew== Message-ID: <26c29e4b-09b1-424a-b4e4-3358aac20115@kernel.org> Date: Wed, 17 Jun 2026 16:36:58 +0200 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 14/15] mm/slab: remove __GFP_NO_OBJ_EXT usage from alloc_slab_obj_exts() 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 , Marco Elver , Dmitry Vyukov , kasan-dev@googlegroups.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org References: <20260615-slab_alloc_flags-v3-0-ce1146d140fb@kernel.org> <20260615-slab_alloc_flags-v3-14-ce1146d140fb@kernel.org> <78b67a9b-44e5-4649-957a-9d42bfaa098e@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/17/26 15:56, Harry Yoo wrote: > > > On 6/15/26 8:54 PM, Vlastimil Babka (SUSE) wrote: >> __GFP_NO_OBJ_EXT has limited scope within the slab allocator itself and >> gfp flags are a scarce resource, unlike slab's alloc_flags. >> >> Introduce SLAB_ALLOC_NO_RECURSE alloc flag that has the same intent as >> __GFP_NO_OBJ_EXT but a more generic name, meaning that a kmalloc() >> family function should not recurse into another kmalloc*() for the >> purposes of allocating auxiliary structures (obj_ext arrays or sheaves). >> >> First, replace the __GFP_NO_OBJ_EXT for allocating obj_ext arrays in >> alloc_slab_obj_exts(). Make use of the newly added kmalloc_flags() >> function, where we can pass alloc_flags with SLAB_ALLOC_NO_RECURSE >> added. This will also pass through SLAB_ALLOC_NOLOCK so we don't need >> to special case kmalloc_nolock() anymore. >> >> Note that until now the kmalloc_nolock() ignored the incoming gfp flags >> and hardcoded __GFP_ZERO | __GFP_NO_OBJ_EXT. But it's correct to pass on >> the incoming gfp flags (only augmented with __GFP_ZERO), because if >> alloc_flags contain SLAB_ALLOC_NOLOCK, the incoming gfp flags have to >> be also compatible with it. However, we might have added __GFP_THISNODE >> for opportunistic slab allocation, as pointed out by Hao Li, and >> __GFP_COMP by allocate_slab() as pointed out by Shengming Hu. Solve this >> by adding both flags to OBJCGS_CLEAR_MASK as it makes sense to strip >> them anyway for non-kmalloc_nolock() allocations of sheaves or obj_ext >> arrays as well. >> >> To avoid recursion of sheaf -> obj_ext -> sheaf -> ... allocations at >> this patch, until the next patch converts sheaves to >> SLAB_ALLOC_NO_RECURSE, use both gfp and alloc_flags for obj_ext. The >> next patch will remove the gfp part. >> >> Link: https://patch.msgid.link/20260610-slab_alloc_flags-v2-15-7190909db118@kernel.org >> Signed-off-by: Vlastimil Babka (SUSE) >> --- > > Looks good to me, > Reviewed-by: Harry Yoo (Oracle) Thanks! > With some comments below. > > I was worried that perhaps replacing SLAB_ALLOC_NO_RECURSE with > __GFP_NO_OBJ_EXT will create a cycle of > > alloc_slab_obj_exts(SLAB_ALLOC_DEFAULT) > -> kmalloc_flags(SLAB_ALLOC_NO_RECURSE) > -> alloc_from_pcs(SLAB_ALLOC_NO_RECURSE) > -> refill_objects(SLAB_ALLOC_DEFAULT) > -> new_slab(SLAB_ALLOC_DEFAULT) > -> account_slab(SLAB_ALLOC_DEFAULT) > -> alloc_slab_obj_exts(SLAB_ALLOC_DEFAULT) > > with __GFP_NO_OBJ_EXT, it would have been passed to refill_objects(), > but SLAB_ALLOC_NO_RECURSE is not. However this cycle does not exist > because alloc_slab_obj_exts() clears __GFP_ACCOUNT (as part of > OBJCG_CLEAR_MASK) and memory profiling itself does not invoke > alloc_slab_obj_exts() when allocating new slabs if SLAB_ACCOUNT is not > set (which is interesting, by the way). Hm yeah I think we should propagate alloc_flags to refill_objects() etc, to avoid later surprise. But can be done as a later cleanup. > Also alloc_slab_obj_exts() propagating SLAB_ALLOC_NEW_SLAB to > kmalloc_flags() is little bit confusing because it does not have any > effect due to SLAB_ALLOC_NO_RECURSE. OK let's address this one by this fixup: diff --git a/mm/slub.c b/mm/slub.c index fc5b8c85b690..dc4b4ae874ce 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -2164,6 +2164,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s, { const bool allow_spin = alloc_flags_allow_spinning(alloc_flags); unsigned int objects = objs_per_slab(s, slab); + bool new_slab = alloc_flags & SLAB_ALLOC_NEW_SLAB; unsigned long new_exts; unsigned long old_exts; struct slabobj_ext *vec; @@ -2173,6 +2174,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s, /* Prevent recursive extension vector allocation */ gfp |= __GFP_NO_OBJ_EXT; alloc_flags |= SLAB_ALLOC_NO_RECURSE; + alloc_flags &= ~SLAB_ALLOC_NEW_SLAB; sz = obj_exts_alloc_size(s, slab, gfp); @@ -2203,7 +2205,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s, old_exts = READ_ONCE(slab->obj_exts); handle_failed_objexts_alloc(old_exts, vec, objects); - if (alloc_flags & SLAB_ALLOC_NEW_SLAB) { + if (new_slab) { /* * If the slab is brand new and nobody can yet access its * obj_exts, no synchronization is required and obj_exts can