From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-180.mta0.migadu.com (out-180.mta0.migadu.com [91.218.175.180]) (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 D58692F5A1F for ; Thu, 23 Oct 2025 08:24:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.180 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761207882; cv=none; b=u2QwKdE6YKgAenjTRp1LMJxPSGhpjhSLy2dH0OJ7x1q3NkYtfcZaMDHKF0xoxccgAPJPcxS9Pp7Zjc2/PfvLLI9trU5XlcoGlEd+6Bizlp+K9Uk8wtpJfxuOVvr5VIKVjY3n26vVnj3N0etNtbozyBkItA1zgHm2RfSQR7ocBYE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761207882; c=relaxed/simple; bh=NcCrlUf4fOwfqPKQ8Z4xxpeURczQzsoS4x4MvBaX9zM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=iTRnKZkL6Q5oxWsg2EwZWfaEuOq4GyIe0QnKTDtGx/kUMMVkX5SIeiF1TLMNumCb1phBI7zSkrQdFEc7KkZhrGlQFXADnY05LLZmTSLD69NvgeYskkdAni46agJqZ1moeqNltPWkcPWi5/H0gYIR8XktO6asI02fuvGMmCYejl4= 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=fThTnFDg; arc=none smtp.client-ip=91.218.175.180 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="fThTnFDg" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1761207877; 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=zxjCR3n53HVShccK/xRoPJo5rB7aVfPPhCgNh53dbY8=; b=fThTnFDg+1ijgoPzAgbubMu5OymL/xwomkEXwN6LQgU2M/qiN/YXap8eaIsyTw5X2NUafZ kCxlvd73b82O3ZmEsS7eXWyV7EXGy9Fsmv90Mm2J23WJIzAYmjTJsnMwgskeLB0U7mPwEY 4D60U5jwltBRaymwIeAk4SGBhPNC1fU= Date: Thu, 23 Oct 2025 16:23:46 +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 15:50, Harry Yoo wrote: > On Thu, Oct 23, 2025 at 11:11:56AM +0800, Hao Ge wrote: >> 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. > Oh, just to be clear I was talking about the other warning: > pr_warn_once("%s, %s: Failed to create slab extension vector!", __func__, s->name); > >> 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 yeah I see what you mean. > > As you mentioned, if the process that failed to allocate the vector wins > cmpxchg(), later process that successfully allocate the vector would > call set_codetag_empty(), so no warning. > > But if the process that allocates the vector wins cmpxchg(), > then it won't call set_codetag_empty(), so the process > that was trying to set OBJEXTS_ALLOC_FAIL now needs to set the tag. Yes, the case I'm encountering is exactly this one. > >>> But anyway, I think there is a better way to do this: > What do you think about the diff I suggested below, though? Sorry for the delayed response earlier; I was trying to deduce all possible scenarios. It makes sense to me, and I will submit the V2 version based on this suggestion. Thank you for your help. > >>> 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; >>> } >>>