+ mm-khugepaged-dont-recycle-vma-pgtable-if-uffd-wp-registered.patch added to -mm tree

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/khugepaged: don't recycle vma pgtable if uffd-wp registered
has been added to the -mm tree.  Its filename is
     mm-khugepaged-dont-recycle-vma-pgtable-if-uffd-wp-registered.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/mm-khugepaged-dont-recycle-vma-pgtable-if-uffd-wp-registered.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/mm-khugepaged-dont-recycle-vma-pgtable-if-uffd-wp-registered.patch

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------------------------------------------------------
From: Peter Xu <peterx@redhat.com>
Subject: mm/khugepaged: don't recycle vma pgtable if uffd-wp registered

When we're trying to collapse a 2M huge shmem page, don't retract pgtable
pmd page if it's registered with uffd-wp, because that pgtable could have
pte markers installed.  Recycling of that pgtable means we'll lose the pte
markers.  That could cause data loss for an uffd-wp enabled application on
shmem.

Instead of disabling khugepaged on these files, simply skip retracting
these special VMAs, then the page cache can still be merged into a huge
thp, and other mm/vma can still map the range of file with a huge thp when
proper.

Note that checking VM_UFFD_WP needs to be done with mmap_sem held for
write, that avoids race like:

         khugepaged                             user thread
         ==========                             ===========
     check VM_UFFD_WP, not set
                                       UFFDIO_REGISTER with uffd-wp on shmem
                                       wr-protect some pages (install markers)
     take mmap_sem write lock
     erase pmd and free pmd page
      --> pte markers are dropped unnoticed!

Link: https://lkml.kernel.org/r/20220405014921.14994-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/khugepaged.c |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

--- a/mm/khugepaged.c~mm-khugepaged-dont-recycle-vma-pgtable-if-uffd-wp-registered
+++ a/mm/khugepaged.c
@@ -1474,6 +1474,10 @@ void collapse_pte_mapped_thp(struct mm_s
 	if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE))
 		return;
 
+	/* Keep pmd pgtable for uffd-wp; see comment in retract_page_tables() */
+	if (userfaultfd_wp(vma))
+		return;
+
 	hpage = find_lock_page(vma->vm_file->f_mapping,
 			       linear_page_index(vma, haddr));
 	if (!hpage)
@@ -1609,7 +1613,15 @@ static void retract_page_tables(struct a
 		 * reverse order. Trylock is a way to avoid deadlock.
 		 */
 		if (mmap_write_trylock(mm)) {
-			if (!khugepaged_test_exit(mm))
+			/*
+			 * When a vma is registered with uffd-wp, we can't
+			 * recycle the pmd pgtable because there can be pte
+			 * markers installed.  Skip it only, so the rest mm/vma
+			 * can still have the same file mapped hugely, however
+			 * it'll always mapped in small page size for uffd-wp
+			 * registered ranges.
+			 */
+			if (!khugepaged_test_exit(mm) && !userfaultfd_wp(vma))
 				collapse_and_free_pmd(mm, vma, addr, pmd);
 			mmap_write_unlock(mm);
 		} else {
_

Patches currently in -mm which might be from peterx@redhat.com are

mm-introduce-pte_marker-swap-entry.patch
mm-teach-core-mm-about-pte-markers.patch
mm-check-against-orig_pte-for-finish_fault.patch
mm-uffd-pte_marker_uffd_wp.patch
mm-shmem-take-care-of-uffdio_copy_mode_wp.patch
mm-shmem-handle-uffd-wp-special-pte-in-page-fault-handler.patch
mm-shmem-persist-uffd-wp-bit-across-zapping-for-file-backed.patch
mm-shmem-allow-uffd-wr-protect-none-pte-for-file-backed-mem.patch
mm-shmem-allows-file-back-mem-to-be-uffd-wr-protected-on-thps.patch
mm-shmem-handle-uffd-wp-during-fork.patch
mm-hugetlb-introduce-huge-pte-version-of-uffd-wp-helpers.patch
mm-hugetlb-hook-page-faults-for-uffd-write-protection.patch
mm-hugetlb-take-care-of-uffdio_copy_mode_wp.patch
mm-hugetlb-handle-uffdio_writeprotect.patch
mm-hugetlb-handle-pte-markers-in-page-faults.patch
mm-hugetlb-allow-uffd-wr-protect-none-ptes.patch
mm-hugetlb-only-drop-uffd-wp-special-pte-if-required.patch
mm-hugetlb-handle-uffd-wp-during-fork.patch
mm-khugepaged-dont-recycle-vma-pgtable-if-uffd-wp-registered.patch
mm-pagemap-recognize-uffd-wp-bit-for-shmem-hugetlbfs.patch
mm-uffd-enable-write-protection-for-shmem-hugetlbfs.patch
mm-enable-pte-markers-by-default.patch
selftests-uffd-enable-uffd-wp-for-shmem-hugetlbfs.patch