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Subject[PATCH v6 19/23] mm/khugepaged: Don't recycle vma pgtable if uffd-wp registered
Date
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!

Signed-off-by: Peter Xu <peterx@redhat.com>
---
mm/khugepaged.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index e99101162f1a..9c75153a36de 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1454,6 +1454,10 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
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)
@@ -1594,7 +1598,15 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
* 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)) {
spinlock_t *ptl = pmd_lock(mm, pmd);
/* assume page table is clear */
_pmd = pmdp_collapse_flush(vma, addr, pmd);
--
2.32.0
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