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 8AD88357CED for ; Mon, 13 Jul 2026 02:41:22 +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=1783910486; cv=none; b=i9dKeNxwCfKRbazLkJZzbz3MxsKFEei7DK90g66duBgMHbX7NUUSClG2AkyGBQaVG8w/T2DWMBrakiWDMwZLZKeJP39m6IY/k4KkVA6s2V141uj+brOvYgZ74wn1DJPa1yRR/wl7One3IswW4JVcrHf4YTKqDmUNM+h4XpBoWkM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783910486; c=relaxed/simple; bh=iGFIKgT0uaCLBYZTrvumuxexPzeRmkumhq1IC8I0lIU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=qLRcUi9W5tH1HIa2p255y0vgkQAeYHl6Ng3pkBXaNZjxAd/9zt/k3LYLe3PpP5DmrGfRipD2my+GhSzDxcxVSnXrcd3PqcVV0GyRjVEIpty+jzf/3A+aOPNPueW6ZYFoJrCFYdcj3pxaem1SzW4MrBuhtwqV8Bk0m0XiCp8wwww= 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=FryoO8/F; 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="FryoO8/F" Message-ID: <4762af9c-e9ac-4162-a407-84d335325dac@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1783910480; 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=2B/xG+qu8furtxmJkYMRRCvbVRz6ayW5kKLZ2HvMBhQ=; b=FryoO8/F6TmYq7NE1embuxGnzVod2pqWmEsa1T0lBvGOnvphTZlNlvZLciBNwUd7hGS2zI /wtvuI7RZYd4dIViTGLaizNgfrmQtQCPTe2kOadIymXh0LQBIv2EUzRby8CQA6QoBLlpCx x4B4xedJDrW/BcH4esObwep8iMEEyec= Date: Mon, 13 Jul 2026 10:40:52 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v5 9/9] mm/page_owner: use memcg_data snapshot instead of PageMemcgKmem() to avoid TOCTOU VM_BUG_ON To: Zi Yan Cc: Vlastimil Babka , Andrew Morton , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260701061101.344679-1-ye.liu@linux.dev> <20260701061101.344679-10-ye.liu@linux.dev> <0E890CCC-818A-47DE-9D1C-9EEF98CC758D@nvidia.com> <4d8e0125-0630-4070-95cd-acec3d626329@linux.dev> <622D6E1C-6E27-4912-A1A8-E5110A4581BB@nvidia.com> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Ye Liu In-Reply-To: <622D6E1C-6E27-4912-A1A8-E5110A4581BB@nvidia.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 在 2026/7/10 23:56, Zi Yan 写道: > On 10 Jul 2026, at 2:51, Ye Liu wrote: > >> 在 2026/7/2 10:02, Zi Yan 写道: >>> On 1 Jul 2026, at 2:10, Ye Liu wrote: >>> >>>> print_page_owner_memcg() takes a snapshot of page->memcg_data via >>>> READ_ONCE at the top of the function and guards against tail pages >>>> and NULL memcg_data. However, at the end it calls PageMemcgKmem(page) >>>> which internally calls folio_memcg_kmem() — and that function re-reads >>>> folio->memcg_data and page->compound_head locklessly, wrapping both >>>> in VM_BUG_ON assertions: >>>> >>>> VM_BUG_ON_PGFLAGS(PageTail(&folio->page), &folio->page); >>>> VM_BUG_ON_FOLIO(folio->memcg_data & MEMCG_DATA_OBJEXTS, folio); >>>> >>>> If the page is concurrently freed and reallocated as a THP tail page >>>> or a slab page between the initial guards and this final call, the >>>> VM_BUG_ON assertions can fire on debug builds (CONFIG_DEBUG_VM=y), >>>> causing a kernel panic. >>>> >>>> Fix by reusing the memcg_data snapshot already taken at function entry >>>> instead of calling PageMemcgKmem(), which is semantically equivalent: >>>> PageMemcgKmem()->folio_memcg_kmem()->folio->memcg_data & MEMCG_DATA_KMEM. >>>> This avoids both the TOCTOU window and the assertions entirely. >>>> >>>> Signed-off-by: Ye Liu >>>> --- >>>> mm/page_owner.c | 2 +- >>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>> >>> LGTM. >>> >>> Reviewed-by: Zi Yan Hi,Zi,Vlastimil >> >> This patch still has warnings (see sashiko link[1]). >> I've made the following modifications, or do you have any better suggestions? > > Maybe use snapshot_page() from mm/debug.c, so that you do not need to replicate > the code from folio_memcg_check(). You probably still need the rcu_read_lock() > when reading memcg_data from the snapshot to prevent memcg going away. > But it does a full struct page + struct folio memcpy with retry loops, which is more than what we need here -- we only care about memcg_data. The READ_ONCE() snapshot is sufficient since the function already holds rcu_read_lock() to keep the memcg alive once resolved. The two-line inline of folio_memcg_check() logic is minimal, and we need the OBJEXTS check to be explicit anyway (to print "Slab cache page\n" -- page_memcg_check() just returns NULL silently for slab pages). Therefore, I don't recommend using snaps_page. Are there any better solutions? >> >> [1]:https://sashiko.dev/#/patchset/20260701061101.344679-1-ye.liu@linux.dev >> >> diff --git a/mm/page_owner.c b/mm/page_owner.c >> index 2e3880053a34..e18512a49e38 100644 >> --- a/mm/page_owner.c >> +++ b/mm/page_owner.c >> @@ -540,6 +540,7 @@ static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret, >> struct page *page) >> { >> unsigned long memcg_data; >> + struct obj_cgroup *objcg; >> struct mem_cgroup *memcg; >> bool online; >> char name[80]; >> @@ -549,11 +550,14 @@ static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret, >> if (!memcg_data || PageTail(page)) >> goto out_unlock; >> >> - if (memcg_data & MEMCG_DATA_OBJEXTS) >> + if (memcg_data & MEMCG_DATA_OBJEXTS) { >> ret += scnprintf(kbuf + ret, count - ret, >> "Slab cache page\n"); >> + goto out_unlock; >> + } >> >> - memcg = page_memcg_check(page); >> + objcg = (void *)(memcg_data & ~OBJEXTS_FLAGS_MASK); >> + memcg = objcg ? obj_cgroup_memcg(objcg) : NULL; >> if (!memcg) >> goto out_unlock; >> >> @@ -561,7 +565,7 @@ static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret, >> cgroup_name(memcg->css.cgroup, name, sizeof(name)); >> ret += scnprintf(kbuf + ret, count - ret, >> "Charged %sto %smemcg %s\n", >> - PageMemcgKmem(page) ? "(via objcg) " : "", >> + (memcg_data & MEMCG_DATA_KMEM) ? "(via objcg) " : "", >> online ? "" : "offline ", >> name); >> out_unlock: >> >>> >>> Best Regards, >>> Yan, Zi >> >> -- >> Thanks, >> Ye Liu > > > Best Regards, > Yan, Zi -- Thanks, Ye Liu