The patch titled Subject: userfaultfd: don't BUG_ON() if khugepaged yanks our page table has been added to the -mm mm-hotfixes-unstable branch. Its filename is userfaultfd-dont-bug_on-if-khugepaged-yanks-our-page-table.patch
This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...
This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's
*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next via the mm-everything branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there every 2-3 working days
------------------------------------------------------ From: Jann Horn jannh@google.com Subject: userfaultfd: don't BUG_ON() if khugepaged yanks our page table Date: Tue, 13 Aug 2024 22:25:22 +0200
Since khugepaged was changed to allow retracting page tables in file mappings without holding the mmap lock, these BUG_ON()s are wrong - get rid of them.
We could also remove the preceding "if (unlikely(...))" block, but then we could reach pte_offset_map_lock() with transhuge pages not just for file mappings but also for anonymous mappings - which would probably be fine but I think is not necessarily expected.
Link: https://lkml.kernel.org/r/20240813-uffd-thp-flip-fix-v2-2-5efa61078a41@googl... Fixes: 1d65b771bc08 ("mm/khugepaged: retract_page_tables() without mmap or vma lock") Signed-off-by: Jann Horn jannh@google.com Reviewed-by: Qi Zheng zhengqi.arch@bytedance.com Acked-by: David Hildenbrand david@redhat.com Cc: Andrea Arcangeli aarcange@redhat.com Cc: Hugh Dickins hughd@google.com Cc: Pavel Emelyanov xemul@virtuozzo.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
mm/userfaultfd.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-)
--- a/mm/userfaultfd.c~userfaultfd-dont-bug_on-if-khugepaged-yanks-our-page-table +++ a/mm/userfaultfd.c @@ -807,9 +807,10 @@ retry: err = -EFAULT; break; } - - BUG_ON(pmd_none(*dst_pmd)); - BUG_ON(pmd_trans_huge(*dst_pmd)); + /* + * For shmem mappings, khugepaged is allowed to remove page + * tables under us; pte_offset_map_lock() will deal with that. + */
err = mfill_atomic_pte(dst_pmd, dst_vma, dst_addr, src_addr, flags, &folio); _
Patches currently in -mm which might be from jannh@google.com are
userfaultfd-fix-checks-for-huge-pmds.patch userfaultfd-dont-bug_on-if-khugepaged-yanks-our-page-table.patch mm-fix-harmless-type-confusion-in-lock_vma_under_rcu.patch