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Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn , Pedro Falcato , Oscar Salvador , Miaohe Lin , Naoya Horiguchi , Adam Litke , Mel Gorman , Rik van Riel , Peter Xu , "Kiryl Shutsemau (Meta)" , "Mike Rapoport (Microsoft)" , Josef Bacik , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: <370ADC80-F15F-40FA-A71E-8D641D060138@linux.dev> References: <20261015-urgent_fixes-v1-0-42796911043b@kernel.org> <20261015-urgent_fixes-v1-2-42796911043b@kernel.org> To: "David Hildenbrand (Arm)" X-Mailer: Apple Mail (2.3901.100.1.1.12) X-Rspamd-Queue-Id: 46592140005 X-Rspam-User: X-Rspamd-Server: rspam12 X-Stat-Signature: b1x1wpaqo4dmd7hgxnqqq9mfpcptur9h X-HE-Tag: 1792135036-460971 X-HE-Meta: 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 uFqoOvLn ua7P6gFwmxQV4lfZqK8LKLxdiKYWN5RSpPTmItH0cO0NUiQnmJSwo5+sW3l4PruggOPqryw0UwlSjWws4iH2TJxpbkMxzyHJC80ZNLi3nQoVqUisHiYvTp4UJF44dF7aGPbXcKmfUyrB7lS+26pUQmAWHm733QoX8DfLJTpWZUCUhlajjdCfRSBJ0HKjCtuisYvUK2CmYnOR7PgbiNVuFq+fHpAQgiYWPVoXKz84I6vinmmI1kgfmiD5hWmCvo3gpIBbWcJKduoEwlbMOikrBp5l5of1c+qIIf3Ak/Of74f7NwBgnXK/MSwTE91bPM3dCSlpKqmEfECiuPRImGHph0mVvQ9IjDYSBAAlmjNRGuKfvH6naWtUAB6n90mE0bfAzwp2IKqzzZnoU0CDEIjLBag0sluPCq/Ho+eyD60X5iN6U0Pw= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: > On Oct 16, 2026, at 00:07, David Hildenbrand (Arm) = wrote: >=20 > From: Josef Bacik >=20 > For a fault in a private mapping with nothing in the page cache, > hugetlb_no_page() allocates a folio, locks it and maps it. The folio = has > one reference, which belongs to the page table entry from then on. = After > dropping the page table lock the function still sets the migratable = flag > of the folio and unlocks it. >=20 > Whoever zaps the entry in between frees the folio. The fault holds = the > fault mutex and the lock of the vma. MADV_DONTNEED and the first = unmap of > a hole punch, hugetlb_vmdelete_list(), do not take the fault mutex, = and a > private vma only has a lock if it owns a reservation map: not with > MAP_NORESERVE, and not in the child after fork(). >=20 > Then the flag is set in, and folio_unlock() is called on, a folio that = is > free or has a new owner. A surplus page goes back to the page = allocator > while it is locked: >=20 > BUG: Bad page state in process init pfn:108e00 > flags: 0x200000000000059(locked|uptodate|dirty|head|node=3D0|zone=3D2) > page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set >=20 > A page of the pool can be given to the next user, of any file, who = locks > it and loses the lock to the late folio_unlock(): >=20 > kernel BUG at mm/filemap.c:1532! > hugetlb_no_page+0x111b/0x23e0 > hugetlb_fault+0x3a9/0x14a0 >=20 > An unprivileged process is enough: threads that write to a MAP_PRIVATE = | > MAP_NORESERVE hugetlb mapping, threads that call MADV_DONTNEED on it, = and > one that takes the freed pages for another file. >=20 > Until commit 3a47c54f09c4 ("hugetlbfs: revert use i_mmap_rwsem for = more > pmd sharing synchronization") the fault held i_mmap_rwsem for reading, > which everybody who zaps takes for writing. The lock that commit > bf4916922c60 ("hugetlbfs: extend hugetlb_vma_lock to private VMAs") = gave > to private vmas closed the window again, but only for those with a > reservation map. >=20 > Be done with the folio before the page table lock is dropped, as > hugetlb_wp() is with its copy. The function already unlocks page = cache > folios under that lock. >=20 > Original fix generated by an LLM and sent in private by Josef. >=20 > Fixes: 3a47c54f09c4 ("hugetlbfs: revert use i_mmap_rwsem for more pmd = sharing synchronization") > Cc: stable@vger.kernel.org > Signed-off-by: Josef Bacik > Reviewed-by: Lorenzo Stoakes (ARM) > [ david: simplify+move comment ] > Signed-off-by: David Hildenbrand (Arm) Acked-by: Muchun Song Thanks.