From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-182.mta1.migadu.com (out-182.mta1.migadu.com [95.215.58.182]) (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 DCAC5286417 for ; Thu, 23 Oct 2025 09:11:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761210722; cv=none; b=EmR50wxQanI7c00Dx5z1Rzy8eGZMtxeuW0tCzjlR22mp3V1/RnEVvf50MyD6j1TTYg+IDcnxx/pMVYrwC93dPDVx7TCos7q9NNjZ0S1PBbw2Dlk3oY6k8HZcQCwhvm/OVQiQXbmwgG45ZOj1D3xfnFYjbeuIwP8WXYjw4blEQBI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761210722; c=relaxed/simple; bh=P6pkN/XT9VOW5uVHhsuwqPpyOs/Klr7TawSDY8T24/w=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=VQsAuspwSBz+dnugy6trsONUiWOYdDQq/++2coh/ApHZeJmQBJEIFEnmkmT+QKzCklwTjJhJ2FKQM0XKQALV/zEUFYW3XkQg4KNY2ii/hWpSS6wYadWKTQ89wltMcJ5MH+K+vxzZWhgrTPVCBfYW+PldFWRgLDXKmK7EnHfryk0= 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=JOkl5iIB; arc=none smtp.client-ip=95.215.58.182 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="JOkl5iIB" Message-ID: <268f8300-3be0-4b45-aa86-e11bf09c86bb@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1761210716; 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=YmispLrE7a4YOiW6K5NNzShTdcLhEQ+ZufQ8+3A6y5g=; b=JOkl5iIBkvZqrgJUDnNLlBLRgFgyo+zC/WeZHufXVg1SvfFYeZX/4spL4xStPG7DSKuy/H dEhLMnEbcXA5S/7nN1CJcjkukdJyic8o2hTsLStMagyJbwTMUitKIvhLvOqXYMtkt5QjOi dZXEH1QcGdmi5lC0JZijQuL2Ze82lcU= Date: Thu, 23 Oct 2025 17:11:01 +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: 8bit X-Migadu-Flow: FLOW_OUT Hi Harry On 2025/10/23 17:06, Harry Yoo wrote: > On Thu, Oct 23, 2025 at 04:46:42PM +0800, Hao Ge wrote: >> Hi Harry >> >> >> On 2025/10/23 16:23, Hao Ge wrote: >>> 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; } >> Maybe it returns false here. >> >> When CONFIG_MEM_ALLOC_PROFILING_DEBUG is not enabled, >> >> The following condition will never be executed: >> >> if (!mark_failed_objexts_alloc(slab) && slab_obj_exts(slab)) > Good point. But without CONFIG_MEM_ALLOC_PROFILING_DEBUG, we don't know > if someone else successfully allocated the vector or not (unlike, with > CONFIG_MEM_ALLOC_PROFILING_DEBUG enabled, we know that when we lose > cmpxchg()). We cannot "fix" the case where a process fails to allocate > the vector but another allocates the vector. > > So I'm not sure if checking slab_obj_exts() once more is worth it in > this case, but I'm fine with either way. > >> if another process that allocates the vector, we will lose one count. > By "one count" you mean skipping accounting the object in memory > profiling, right? Yes. > >>>>>>    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; >>>>>>        } >>>>>>