[PATCH v4 1/2] mm/khugepaged: Never install PMDs in uffd-minor-registered VMAs

[James Houghton <jthoughton@google.com>]

Userfaultfd minor faults provides userspace with the ability to manually
install PTEs with UFFDIO_CONTINUE. Right now, MADV_COLLAPSE can map
holes in the VMA when a naturally-aligned THP is present. This is not
true for khugepaged collapse: the PTEs will be retracted, but a PMD will
not be installed.

When MADV_COLLAPSE installs a PMD that mapped holes in the VMA,
userspace is likely to expect UFFDIO_CONTINUE to succeed on the
should-be holes. UFFDIO_CONTINUE will fail and return EEXIST.

This is not inherently a problem, as MADV_COLLAPSE is an explicit
userspace action. But, especially because MADV_COLLAPSE can be invoked
by an external process via process_madvise(), a rogue caller could break
a userfaultfd-minor resolver thread.

Userspace cannot generally use MADV_COLLAPSE to resolve userfault minor
faults, as MADV_COLLAPSE will only resolve such faults if a
naturally-aligned THP is present, so this is not a functional
regression for userspace.

The naturally-aligned THP case is the only case where this quirk exists.
Collapsing otherwise requires all PTEs to be present for
userfaultfd-registered VMAs (i.e., max none PTEs is 0), which is
correct. This check is essentially bypassed for naturally-aligned THPs.

Suggested-by: Lance Yang <lance.yang@linux.dev>
Tested-by: Lance Yang <lance.yang@linux.dev>
Signed-off-by: James Houghton <jthoughton@google.com>
---
Changes since v3:
  - Prevent page table retraction in patch 1. Please see the comment
    next to it. So this patch now has two hunks instead of one.
  - Reworded the comment and combined the userfaultfd checks in patch 1.
    (Thanks David)
  - Simplified the selftest a bit given the behavior change in patch 1.
  - Undid a change in v3's selftest that incorrectly handled the case
    where MADV_COLLAPSE was not supported entirely.
  - Rebased on top of mm-unstable, which includes Kiryl's changes.
v3: https://lore.kernel.org/linux-mm/20260910023411.514987-1-jthoughton@google.com/
v2: https://lore.kernel.org/linux-mm/20260828222640.1638457-1-jthoughton@google.com/
v1: https://lore.kernel.org/linux-mm/20260828005004.2870750-1-jthoughton@google.com/
---
 mm/khugepaged.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 913086eaf17b..438c4af1c058 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1868,10 +1868,11 @@ static enum scan_result try_collapse_pte_mapped_thp(struct mm_struct *mm, unsign
 		return SCAN_VMA_CHECK;
 
 	/*
-	 * Keep pmd pgtable while the uffd bit is in use; see comment in
-	 * retract_page_tables().
+	 * Don't collapse if there might be PTE markers for userfaultfd-based
+	 * access protection or if collapsing might bypass userfaultfd minor
+	 * faults.
 	 */
-	if (userfaultfd_protected(vma))
+	if (userfaultfd_protected(vma) || userfaultfd_minor(vma))
 		return SCAN_PTE_UFFD;
 
 	folio = filemap_lock_folio(vma->vm_file->f_mapping,
@@ -2093,8 +2094,13 @@ static bool file_backed_vma_is_retractable(struct vm_area_struct *vma)
 	 * and cannot be recycled to a shared PMD. Other vmas can still
 	 * have the same file mapped hugely, but skip this one: it will
 	 * always be mapped in small page size for these registrations.
+	 *
+	 * Userfaultfd-minor-registered VMAs should also not be retracted.
+	 * PMDs will not be installed, as doing so can suppress minor faults.
+	 * If retraction were allowed, khugepaged might continually cause
+	 * unnecessary userfaultfd minor faults for already-CONTINUE'd pages.
 	 */
-	if (userfaultfd_protected(vma))
+	if (userfaultfd_protected(vma) || userfaultfd_minor(vma))
 		return false;
 
 	/*

base-commit: b2b4b29b76dabdee576eba66953a66ca61c5fca0
-- 
2.56.0.rc1.315.gc6ed9934b7-goog