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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 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 X-Rspamd-Queue-Id: 415B620007 X-Stat-Signature: b4h1jsspn86hhzdd7czehecemzu5gprp X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1783964089-966723 X-HE-Meta: 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 lR9pqAE1 fnhs1pKtWS5u6hoykUtUGFTLOjRARWddFbdO93hZgFfXPh+fH7RhNILqUd3cxlOx5buzN5Z0sg//KqtXwqNaO3+3c9GgcvibE+pwr/0JG+cbIUaLzslOv6qD3HaudL9mQcZEgfnKq9CMWanK4I54U7sTLiFVb/ZjvV7UaQvAIFowcbMgrtn7M02voMuiyMd3QHAuAdtCGLMjSBKDvc7Xmns4Cf3704YrQoCEY19rTEUnJcf5kIC7A06GQT+0j3S2DVuYDMxvuLEC6CGQ7Qa0IM9wyfjTP09/8nxkVh+jdn7695EJO8c0M4TKUuXFMI+bDoZMoS0Ygc0b/szBG91HB+7+vwzEsqU/900D0 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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.