+ mm-hugetlb-pre-allocate-pgtable-pages-for-uffd-wr-protects.patch added to mm-hotfixes-unstable branch

+ mm-hugetlb-pre-allocate-pgtable-pages-for-uffd-wr-protects.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/hugetlb: pre-allocate pgtable pages for uffd wr-protects
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-hugetlb-pre-allocate-pgtable-pages-for-uffd-wr-protects.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-hugetlb-pre-allocate-pgtable-pages-for-uffd-wr-protects.patch

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

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------------------------------------------------------
From: Peter Xu <peterx@redhat.com>
Subject: mm/hugetlb: pre-allocate pgtable pages for uffd wr-protects
Date: Wed, 4 Jan 2023 17:52:05 -0500

Userfaultfd-wp uses pte markers to mark wr-protected pages for both shmem
and hugetlb.  Shmem has pre-allocation ready for markers, but hugetlb path
was overlooked.

Doing so by calling huge_pte_alloc() if the initial pgtable walk fails to
find the huge ptep.  It's possible that huge_pte_alloc() can fail with
high memory pressure, in that case stop the loop immediately and fail
silently.  This is not the most ideal solution but it matches with what we
do with shmem meanwhile it avoids the splat in dmesg.

Link: https://lkml.kernel.org/r/20230104225207.1066932-2-peterx@redhat.com
Fixes: 60dfaad65aa9 ("mm/hugetlb: allow uffd wr-protect none ptes")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reported-by: James Houghton <jthoughton@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: <stable@vger.kernel.org>	[5.19+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/hugetlb.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

--- a/mm/hugetlb.c~mm-hugetlb-pre-allocate-pgtable-pages-for-uffd-wr-protects
+++ a/mm/hugetlb.c
@@ -6660,8 +6660,17 @@ unsigned long hugetlb_change_protection(
 		spinlock_t *ptl;
 		ptep = huge_pte_offset(mm, address, psize);
 		if (!ptep) {
-			address |= last_addr_mask;
-			continue;
+			if (!uffd_wp) {
+				address |= last_addr_mask;
+				continue;
+			}
+			/*
+			 * Userfaultfd wr-protect requires pgtable
+			 * pre-allocations to install pte markers.
+			 */
+			ptep = huge_pte_alloc(mm, vma, address, psize);
+			if (!ptep)
+				break;
 		}
 		ptl = huge_pte_lock(h, mm, ptep);
 		if (huge_pmd_unshare(mm, vma, address, ptep)) {
_

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

mm-hugetlb-pre-allocate-pgtable-pages-for-uffd-wr-protects.patch
mm-uffd-fix-pte-marker-when-fork-without-fork-event.patch
mm-fix-a-few-rare-cases-of-using-swapin-error-pte-marker.patch
mm-uffd-always-wr-protect-pte-in-ptepmd_mkuffd_wp.patch
mm-hugetlb-let-vma_offset_start-to-return-start.patch
mm-hugetlb-dont-wait-for-migration-entry-during-follow-page.patch
mm-hugetlb-document-huge_pte_offset-usage.patch
mm-hugetlb-move-swap-entry-handling-into-vma-lock-when-faulted.patch
mm-hugetlb-make-userfaultfd_huge_must_wait-safe-to-pmd-unshare.patch
mm-hugetlb-make-hugetlb_follow_page_mask-safe-to-pmd-unshare.patch
mm-hugetlb-make-follow_hugetlb_page-safe-to-pmd-unshare.patch
mm-hugetlb-make-walk_hugetlb_range-safe-to-pmd-unshare.patch
mm-hugetlb-introduce-hugetlb_walk.patch
mm-mprotect-use-long-for-page-accountings-and-retval.patch
mm-uffd-detect-pgtable-allocation-failures.patch