From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-181.mta1.migadu.com (out-181.mta1.migadu.com [95.215.58.181]) (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 D25D213D2B2 for ; Thu, 23 Oct 2025 03:12:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.181 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761189180; cv=none; b=DqkcaqDufPghuApOSPfM2uUkZi/gQpWpo+6NMt71n0wSb2m2/uKiTLNgujh5s1DUchECHmglmjrl8Pood6ywXvt1pZ9YIydyNf4KE9QEW/Q9kQkfT1avLItJkUzGaudi+HJYiPLVZR8K3S9wOitMm74XJj6jHJrIekllq1GDk6k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761189180; c=relaxed/simple; bh=L5JoeUi4WeEe+pRIOovU+B+uZJIWZX7XGgvaKQi3ENY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Dkz3gz+TfKlk++Sew5qcCJx767+w/boij6QwUXowZpLsCa4vZ4TKub1BD1QEla3vGinazzWld5XqDTgSVUNi/erFj2jrZXU2PWPl4mtq4FYrnoErEnfzo4hbQipgePp6HoK/EgnKU92wzPCE0H5rcfBXi0q/mLTK0ytrldQ/Qfo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=SJMdsd/v; arc=none smtp.client-ip=95.215.58.181 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="SJMdsd/v" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1761189174; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=LFkWNTRoB/vWjqG2R3wUjVcnbHUc9YIZUBF8y0lylxA=; b=SJMdsd/v/n7qYpNMD0GsCkMe2XJR8wOEzvSvdCFiCAlKhs6jakoNH86bSAuk2ZTSAVA1yr mcpeQWoWfK0diJ92k0LlQzJPP7wRBJpvB7HdKx/kYWSEsmUfrX5aLKxZp1WMlBzEH+RFZO OI9eYVdTtQKpPqONpyuPavmZfqmz8AM= Date: Thu, 23 Oct 2025 11:11:56 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH] slab: Fix obj_ext is mistakenly considered NULL due to race condition To: Harry Yoo Cc: Vlastimil Babka , Andrew Morton , Christoph Lameter , David Rientjes , Roman Gushchin , Suren Baghdasaryan , Shakeel Butt , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hao Ge References: <20251023012117.890883-1-hao.ge@linux.dev> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Hao Ge In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT Hi Harry On 2025/10/23 10:24, Harry Yoo wrote: > On Thu, Oct 23, 2025 at 09:21:17AM +0800, Hao Ge wrote: >> From: Hao Ge >> >> If two competing threads enter alloc_slab_obj_exts(), and the >> thread that failed to allocate the object extension vector exits >> after the one that succeeded, it will mistakenly assume slab->obj_ext >> is still empty due to its own allocation failure. This will then trigger >> warnings enforced by CONFIG_MEM_ALLOC_PROFILING_DEBUG checks in >> the subsequent free path. >> >> Therefore, let's add an additional check when alloc_slab_obj_exts fails. >> >> Signed-off-by: Hao Ge >> --- >> mm/slub.c | 9 ++++++--- >> 1 file changed, 6 insertions(+), 3 deletions(-) >> >> diff --git a/mm/slub.c b/mm/slub.c >> index d4403341c9df..42276f0cc920 100644 >> --- a/mm/slub.c >> +++ b/mm/slub.c >> @@ -2227,9 +2227,12 @@ prepare_slab_obj_exts_hook(struct kmem_cache *s, gfp_t flags, void *p) >> slab = virt_to_slab(p); >> if (!slab_obj_exts(slab) && >> alloc_slab_obj_exts(slab, s, flags, false)) { >> - pr_warn_once("%s, %s: Failed to create slab extension vector!\n", >> - __func__, s->name); >> - return NULL; >> + /* Recheck if a racing thread has successfully allocated slab->obj_exts. */ >> + if (!slab_obj_exts(slab)) { >> + pr_warn_once("%s, %s: Failed to create slab extension vector!\n", >> + __func__, s->name); >> + return NULL; >> + } >> } > Maybe this patch is a bit paranoid... since if mark_failed_objexts_alloc() > win cmpxchg() and then someone else allocates the object extension vector, > the warning will still be printed anyway. The process that successfully allocates slab_exts will call handle_failed_objexts_alloc, setting ref->ct = CODETAG_EMPTY to prevent the warning from being triggered. > But anyway, I think there is a better way to do this: > > diff --git a/mm/slub.c b/mm/slub.c > index dd4c85ea1038..d08d7580349d 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -2052,9 +2052,9 @@ static inline void mark_objexts_empty(struct slabobj_ext *obj_exts) > } > } > > -static inline void mark_failed_objexts_alloc(struct slab *slab) > +static inline bool mark_failed_objexts_alloc(struct slab *slab) > { > - cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL); > + return cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL) == 0; > } > > static inline void handle_failed_objexts_alloc(unsigned long obj_exts, > @@ -2076,7 +2076,7 @@ static inline void handle_failed_objexts_alloc(unsigned long obj_exts, > #else /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */ > > static inline void mark_objexts_empty(struct slabobj_ext *obj_exts) {} > -static inline void mark_failed_objexts_alloc(struct slab *slab) {} > +static inline bool mark_failed_objexts_alloc(struct slab *slab) { return true; } > static inline void handle_failed_objexts_alloc(unsigned long obj_exts, > struct slabobj_ext *vec, unsigned int objects) {} > > @@ -2125,7 +2125,9 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s, > } > if (!vec) { > /* Mark vectors which failed to allocate */ > - mark_failed_objexts_alloc(slab); > + if (!mark_failed_objexts_alloc(slab) && > + slab_obj_exts(slab)) > + return 0; > > return -ENOMEM; > } >