From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 2EBE434CFC3 for ; Mon, 13 Jul 2026 17:34:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783964089; cv=none; b=lZcR3T/YlJsCtJY4bVkf7IuCyGPzIfO9k7BfXL/LX2WQ/TRjdEj26o4wE5Qx0bG4pyUw0IGrKYOE02BTU9s8wKJ7CZEamw+i4ROx2EoLd5esV3vu4rLWWTSnw9SN6zz/jLFmqdGCzV7Y8uhjGlPYgTLg4ojJu7iHgwNrGkSneV4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783964089; c=relaxed/simple; bh=3Bw9xfG2tEcdRNEeE7IjliOaCy7u1B+nYYAwxBij6qQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=JAwlvFdINHIE/Nmec+vuSdTksoEZGdSidPn9r2ZYnx8hc81htTkCqJdEZiY2kz19J+hy5zqstC9tQp8qTooCUyhS54bvzO6okpfB4t0wHUOqh7CphuMSJacz7x8BEdEQY7aucxko+nyRCdG0o0fmDkJMskiOW7Vhe1S3clao3xM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=I0eAvfdS; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="I0eAvfdS" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 551A21F000E9; Mon, 13 Jul 2026 17:34:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783964088; bh=H9z7EIoWx2sCC/Q8euoaMwL2ld9Y9xIG46XmvenPEPE=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=I0eAvfdSe9fzx/FK4W2PTqf/haS26znBzde5uSBglJCcC9DFp1+olacfpXid69CHm Qw6jgEitzxtXNqU3CJHpAvSxrLHIDG48PbgGPKJp0V+elP68HrHauODRcz163qWYF0 UNZmK9vRDgzIbsC46KYpIcx47nscFuo8dxDri6in+m6ZyK7bpackuJzcN7KDN4iOZv vuJDZ0IArQ1dQJFzA27rBLWL+4rSB6bPunvG+vhbcnjuQuDhS0oDiG4t9clVK2NqYG 9qsD2L7yVTYXFDadso1IrPhP25bdGQsizYg7vtZE4hd+VDM5ML8FQlN58t9NSrJx0L XF/RoCQ0UvPmg== Message-ID: <48dd2c46-bf55-44fe-b7e6-24037ba59aed@kernel.org> Date: Mon, 13 Jul 2026 19:34:44 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 9/9] mm/page_owner: use memcg_data snapshot instead of PageMemcgKmem() to avoid TOCTOU VM_BUG_ON Content-Language: en-US To: Ye Liu , Zi Yan Cc: 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> <4762af9c-e9ac-4162-a407-84d335325dac@linux.dev> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 7/13/26 04:40, Ye Liu wrote: > > > 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. Yeah seems too unnecessarily heavy. > 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). Yeah LGTM.