[Qemu-devel] [PATCH 09/23] userfaultfd: prevent khugepaged to merge if userfaultfd is armed

[Andrea Arcangeli <aarcange@redhat.com>]

If userfaultfd is armed on a certain vma we can't "fill" the holes
with zeroes or we'll break the userland on demand paging. The holes if
the userfault is armed, are really missing information (not zeroes)
that the userland has to load from network or elsewhere.

The same issue happens for wrprotected ptes that we can't just convert
into a single writable pmd_trans_huge.

We could however in theory still merge across zeropages if only
VM_UFFD_MISSING is set (so if VM_UFFD_WP is not set)... that could be
slightly improved but it'd be much more complex code for a tiny corner
case.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---
 mm/huge_memory.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index c221be3..9671f51 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2198,7 +2198,8 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 	     _pte++, address += PAGE_SIZE) {
 		pte_t pteval = *_pte;
 		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
-			if (++none_or_zero <= khugepaged_max_ptes_none)
+			if (!userfaultfd_armed(vma) &&
+			    ++none_or_zero <= khugepaged_max_ptes_none)
 				continue;
 			else
 				goto out;
@@ -2651,7 +2652,8 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
 	     _pte++, _address += PAGE_SIZE) {
 		pte_t pteval = *_pte;
 		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
-			if (++none_or_zero <= khugepaged_max_ptes_none)
+			if (!userfaultfd_armed(vma) &&
+			    ++none_or_zero <= khugepaged_max_ptes_none)
 				continue;
 			else
 				goto out_unmap;