+ userfaultfd-dont-bug_on-if-khugepaged-yanks-our-page-table.patch added to mm-hotfixes-unstable branch

+ userfaultfd-dont-bug_on-if-khugepaged-yanks-our-page-table.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

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/userfaultfd-dont-bug_on-if-khugepaged-yanks-our-page-table.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
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@google.com
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