+ mm-mprotect-dont-skip-uffd-marker-install-when-racing-with-pte-table-reclaim.patch added to mm-new branch

+ mm-mprotect-dont-skip-uffd-marker-install-when-racing-with-pte-table-reclaim.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm/mprotect: don't skip uffd marker install when racing with PTE table reclaim
has been added to the -mm mm-new branch.  Its filename is
     mm-mprotect-dont-skip-uffd-marker-install-when-racing-with-pte-table-reclaim.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-mprotect-dont-skip-uffd-marker-install-when-racing-with-pte-table-reclaim.patch

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

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

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and is updated there most days

------------------------------------------------------
From: "David Hildenbrand (Arm)" <david@kernel.org>
Subject: mm/mprotect: don't skip uffd marker install when racing with PTE table reclaim
Date: Sat, 07 Nov 2026 00:28:55 +0100

change_pmd_range() populates PTE tables before installing uffd-wp markers.
An empty PTE table can be removed after change_pmd_prepare() returns but
before the pmd_none() check.

That check then skips the range, allowing UFFDIO_WRITEPROTECT to report
success without installing the required markers.  Subsequent writes by
userspace to these areas will not trigger uffd events.

PTE table reclaim during MADV_DONTNEED and page table retraction during
khugepaged can trigger this.

Retry page-table allocation in that case. Simplify the code, getting
rid of change_pmd_prepare() by routing all code through the "again" label.

It's a small race though, so likely this is not very urgent.

Link: https://lore.kernel.org/20261107-uffd_shmem_marker-v1-1-91fa2d653cfe@kernel.org
Fixes: fe2567eb5521 ("mm/shmem: allow uffd wr-protect none pte for file-backed mem")
Fixes: 6375e95f381e ("mm: pgtable: reclaim empty PTE page in madvise(MADV_DONTNEED)")
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: Tal Zussman <tz2294@columbia.edu>
Cc: <stable@vger.kernel.org>
---

 mm/mprotect.c |   48 +++++++++++-------------------------------------
 1 file changed, 11 insertions(+), 37 deletions(-)

--- a/mm/mprotect.c~mm-mprotect-dont-skip-uffd-marker-install-when-racing-with-pte-table-reclaim
+++ a/mm/mprotect.c
@@ -470,25 +470,8 @@ pgtable_populate_needed(struct vm_area_s
 }
 
 /*
- * Populate the pgtable underneath for whatever reason if requested.
- * When {pte|pmd|...}_alloc() failed we treat it the same way as pgtable
- * allocation failures during page faults by kicking OOM and returning
- * error.
- */
-#define  change_pmd_prepare(vma, pmd, cp_flags)				\
-	({								\
-		long err = 0;						\
-		if (unlikely(pgtable_populate_needed(vma, cp_flags))) {	\
-			if (pte_alloc(vma->vm_mm, pmd))			\
-				err = -ENOMEM;				\
-		}							\
-		err;							\
-	})
-
-/*
- * This is the general pud/p4d/pgd version of change_pmd_prepare(). We need to
- * have separate change_pmd_prepare() because pte_alloc() returns 0 on success,
- * while {pmd|pud|p4d}_alloc() returns the valid pointer on success.
+ * Populate the lower-level page table if marker installation requires it.
+ * Return -ENOMEM on allocation failure.
  */
 #define  change_prepare(vma, high, low, addr, cp_flags)			\
 	  ({								\
@@ -517,30 +500,21 @@ static inline long change_pmd_range(stru
 again:
 		next = pmd_addr_end(addr, end);
 
-		ret = change_pmd_prepare(vma, pmd, cp_flags);
-		if (ret) {
-			pages = ret;
-			break;
-		}
-
-		if (pmd_none(*pmd))
+		if (unlikely(pgtable_populate_needed(vma, cp_flags))) {
+			if (pte_alloc(vma->vm_mm, pmd)) {
+				pages = -ENOMEM;
+				break;
+			}
+		} else if (pmd_none(*pmd)) {
 			goto next;
+		}
 
 		_pmd = pmdp_get_lockless(pmd);
 		if (pmd_is_huge(_pmd)) {
 			if ((next - addr != HPAGE_PMD_SIZE) ||
 			    pgtable_split_needed(vma, cp_flags)) {
 				__split_huge_pmd(vma, pmd, addr, false);
-				/*
-				 * For file-backed, the pmd could have been
-				 * cleared; make sure pmd populated if
-				 * necessary, then fall-through to pte level.
-				 */
-				ret = change_pmd_prepare(vma, pmd, cp_flags);
-				if (ret) {
-					pages = ret;
-					break;
-				}
+				goto again;
 			} else {
 				ret = change_huge_pmd(tlb, vma, pmd,
 						addr, newprot, cp_flags);
@@ -554,7 +528,7 @@ again:
 					goto next;
 				}
 			}
-			/* fall through, the trans huge pmd just split */
+			/* The PMD might have been split. */
 		}
 
 		ret = change_pte_range(tlb, vma, pmd, addr, next, newprot,
_

Patches currently in -mm which might be from david@kernel.org are

mm-memory-failure-fix-folio-refcount-leak-and-min_order_for_split-locking.patch
mm-free-unused-page-table-allocated-by-do_set_pmd.patch
mm-dont-allocate-a-page-table-under-rcu-in-do_set_pmd.patch
mm-mprotect-dont-skip-uffd-marker-install-when-racing-with-pte-table-reclaim.patch