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From: Muchun Song In-Reply-To: <20250108074822.722696-1-yuzhao@google.com> Date: Thu, 9 Jan 2025 10:40:26 +0800 Cc: Andrew Morton , David Hildenbrand , Mateusz Guzik , "Matthew Wilcox (Oracle)" , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Will Deacon Content-Transfer-Encoding: quoted-printable Message-Id: <4D77AD25-6DC2-406D-9D54-9032F38260B4@linux.dev> References: <20250108074822.722696-1-yuzhao@google.com> To: Yu Zhao X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 54DC7C0006 X-Stat-Signature: g4tcrbdpcu4p9fcnke3suqy8iijdhrpu X-Rspam-User: X-HE-Tag: 1736390468-168655 X-HE-Meta: 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 bAnCmhDh NwqtVSTN1xQAltQlic78T7U0wyNCspaNbfm/CdUVKpKEesw9D01KOGsnN1fsFBbFElIjXsLbEsxwHz5OZic7qJpOh1b7MLI0tFUIx7hJFro+g5ecsscAiMzVu71X9cu7rMlvUZ4qsMbyJVAxsE0ikA1emR+7SbvdGGqWesl9ecvf2faN7+Fe5TRYat2sf1OpMSIobIGCglyZ86SsIfnWh7n3nUnKCL+ZHZaSgKoq/VfGXt/8Y4lRKzTXf8ONJyjSi8OVsXXsZWILJ49g= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: > On Jan 8, 2025, at 15:48, Yu Zhao wrote: >=20 > Using x86_64 as an example, for a 32KB struct page[] area describing a > 2MB hugeTLB, HVO reduces the area to 4KB by the following steps: > 1. Split the (r/w vmemmap) PMD mapping the area into 512 (r/w) PTEs; > 2. For the 8 PTEs mapping the area, remap PTE 1-7 to the page mapped > by PTE 0, and at the same time change the permission from r/w to > r/o; > 3. Free the pages PTE 1-7 used to map, hence the reduction from 32KB > to 4KB. >=20 > However, the following race can happen due to improperly memory loads > ordering: > CPU 1 (HVO) CPU 2 (speculative PFN walker) >=20 > page_ref_freeze() > synchronize_rcu() > rcu_read_lock() > page_is_fake_head() is false > vmemmap_remap_pte() > XXX: struct page[] becomes r/o >=20 > page_ref_unfreeze() > page_ref_count() is not zero >=20 > atomic_add_unless(&page->_refcount) > XXX: try to modify r/o struct page[] >=20 > Specifically, page_is_fake_head() must be ordered after > page_ref_count() on CPU 2 so that it can only return true for this > case, to avoid the later attempt to modify r/o struct page[]. >=20 > This patch adds the missing memory barrier and makes the tests on > page_is_fake_head() and page_ref_count() done in the proper order. >=20 > Fixes: bd225530a4c7 ("mm/hugetlb_vmemmap: fix race with speculative = PFN walkers") > Reported-by: Will Deacon > Closes: = https://lore.kernel.org/20241128142028.GA3506@willie-the-truck/ > Signed-off-by: Yu Zhao Reviewed-by: Muchun Song Thanks.