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 20FDF44BC97; Wed, 17 Jun 2026 14:57:03 +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=1781708226; cv=none; b=XYdTvJYGmS32bHM1dj0vCqVSDLnZI5HLqIKJ5lIXGOaJyXicg4H/pUG47ysxDDQ4krCtv+ltuyQK7r9+7ipvTd23kr4phHDWuJyFIFiNUgVh1Cz/Xq6DSkCJt0pdbTGjEFaL5ed+rS7x+GyetBUHXFVT5dd6pHT6UoLNRFvsvHc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781708226; c=relaxed/simple; bh=wmM6sI66JIelazf0lQlD2kB+QRS5EgACQWkLhjdJt2g=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=tYDCIIWD3W/DjS4IV+1OxO/ty/keoYBXcJORy3hLo7cmtV8gApCbmOwcmGYzXanljw1uRxA0x7dP9ZKWy/lD8Nbb63eThb9CT45P8ZY/xI0N1t6AqpanI40K7+sqxb/mKuS9b5VAAHe/Cebg5ghjahFMWK5QewjA+u+NdtucJXU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=XgFp1k9P; 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="XgFp1k9P" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 451D51F000E9; Wed, 17 Jun 2026 14:56:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781708223; bh=JrfGjOmOPJXcDh++I3CnLLhJP9P9yWCrcpWYJkVADXo=; h=Date:Subject:From:To:Cc:References:In-Reply-To; b=XgFp1k9P/dScG/0MwNL3xeu3uWnqakesjXiQFGSgyEt3sIBFxG92nv/pZKKSRsUIJ Svg5oTYzHqshNPe6Bf937iNLaGYCcLXOU/owaCO3Fbt3atKKVFEHWIffFo/kUVBWSc rHVvDzp6BrCUbJm6DW9utKp2bLcMSCfO7PxZ4xoNc4hJb/+Yz/zTJuu3mb1iZNCKWH hgGym3kRiqcsOuN+7XR7Av1KKkpGeNaewcK0a6hi3J3BRk+iG9dnywc0jaYn7xGPKJ biElo+NdDsqqsWPFvAnxEU5HW1xnthPmSMu0iXVcWyaUIUj6LcxF14fiPPg7tMw/En F/0DhwG/BRaXg== Message-ID: <1bf749a4-1519-4d14-a0a7-6d8a56a6c850@kernel.org> Date: Wed, 17 Jun 2026 16:56:58 +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 v3 14/15] mm/slab: remove __GFP_NO_OBJ_EXT usage from alloc_slab_obj_exts() Content-Language: en-US From: "Vlastimil Babka (SUSE)" 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> <26c29e4b-09b1-424a-b4e4-3358aac20115@kernel.org> Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/17/26 16:36, Vlastimil Babka (SUSE) wrote: > >> 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. It's also not a new hazard I think because while previously gfp flags with __GFP_NO_OBJ_EXT would could be propagated more thoroughly than alloc_flags for obj_exts only __alloc_tagging_slab_alloc_hook() looks at them, and alloc_slab_obj_exts() (from account_slab()) didn't either, so the amount of (finite) recursion is the same I think. >> 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 >