From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-188.mta1.migadu.com (out-188.mta1.migadu.com [95.215.58.188]) (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 6A532302177 for ; Thu, 23 Oct 2025 08:47:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.188 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761209271; cv=none; b=XIucfToeU8AUrDLoXicu5njy57KPTUaaH+bEu4Y2HSH4iu3/Fu1EMLwO0w8lmVWEx+K9wy9q+YnAB8iMCXYz9KxO44dRrIwnt5ocB805d7UybReBKxW7QGSHLKFCzgcwi3fq1PZhu/JmxvEoNEby8HcrAUQYMG4rGIQQv9OScjc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761209271; c=relaxed/simple; bh=i3ZYv34TrEQuJCuvjlYNf7aY4h5bQm00aG9u6+rYQ/c=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=NvVT4/NcTGki16eZt+LqJ0x4PCGDvfE6s2G34ekh/WH8LnUVpzecpOhz28kmy7goxj3VGIGFzvAsV/Bfxw17Kp/R0s+JSW9wKIgPQ9Q4zY6qLZLZUM3cDauzDamMGtt9etJFLCgP9zEnpLHLSbWHAQN99uMgL9+R4TR4IUzarvc= 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=iHF5CACX; arc=none smtp.client-ip=95.215.58.188 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="iHF5CACX" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1761209265; 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=5CO9JUnFW5uKy5VvVF8W1FG0awi4JoMoz5N1pXPIqZU=; b=iHF5CACXnrJQQWiRMI1Yp5/2Be2BFxIX8fXmBkMFWrJADJckfgPzF/ig7uhZWykPqfLWM0 Zc0X1L6GlGjGTEOSiVST+Bz5PJWy5gbA1jQhqIaD1/ymBJ/HTQYEBIf1sPBpoGqO/iRlbP g8wN1v/jVa14BXdLvLLEaQMfrSRc8P4= Date: Thu, 23 Oct 2025 16:46:42 +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 X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Hao Ge 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 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 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)) if another process that allocates the vector, we will lose one count. >>>>    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; >>>>        } >>>>