+ mm-hugetlb-make-detecting-shared-pte-more-reliable.patch added to mm-unstable branch

+ mm-hugetlb-make-detecting-shared-pte-more-reliable.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/hugetlb: make detecting shared pte more reliable
has been added to the -mm mm-unstable branch.  Its filename is
     mm-hugetlb-make-detecting-shared-pte-more-reliable.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-hugetlb-make-detecting-shared-pte-more-reliable.patch

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

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------------------------------------------------------
From: Miaohe Lin <linmiaohe@huawei.com>
Subject: mm/hugetlb: make detecting shared pte more reliable
Date: Tue, 16 Aug 2022 21:05:53 +0800

If the pagetables are shared, we shouldn't copy or take references.  Since
src could have unshared and dst shares with another vma, huge_pte_none()
is thus used to determine whether dst_pte is shared.  But this check isn't
reliable.  A shared pte could have pte none in pagetable in fact.  The
page count of ptep page should be checked here in order to reliably
determine whether pte is shared.

Link: https://lkml.kernel.org/r/20220816130553.31406-7-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/hugetlb.c |   16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

--- a/mm/hugetlb.c~mm-hugetlb-make-detecting-shared-pte-more-reliable
+++ a/mm/hugetlb.c
@@ -4795,15 +4795,13 @@ int copy_hugetlb_page_range(struct mm_st
 
 		/*
 		 * If the pagetables are shared don't copy or take references.
-		 * dst_pte == src_pte is the common case of src/dest sharing.
 		 *
+		 * dst_pte == src_pte is the common case of src/dest sharing.
 		 * However, src could have 'unshared' and dst shares with
-		 * another vma.  If dst_pte !none, this implies sharing.
-		 * Check here before taking page table lock, and once again
-		 * after taking the lock below.
+		 * another vma. So page_count of ptep page is checked instead
+		 * to reliably determine whether pte is shared.
 		 */
-		dst_entry = huge_ptep_get(dst_pte);
-		if ((dst_pte == src_pte) || !huge_pte_none(dst_entry)) {
+		if (page_count(virt_to_page(dst_pte)) > 1) {
 			addr |= last_addr_mask;
 			continue;
 		}
@@ -4814,11 +4812,9 @@ int copy_hugetlb_page_range(struct mm_st
 		entry = huge_ptep_get(src_pte);
 		dst_entry = huge_ptep_get(dst_pte);
 again:
-		if (huge_pte_none(entry) || !huge_pte_none(dst_entry)) {
+		if (huge_pte_none(entry)) {
 			/*
-			 * Skip if src entry none.  Also, skip in the
-			 * unlikely case dst entry !none as this implies
-			 * sharing with another vma.
+			 * Skip if src entry none.
 			 */
 			;
 		} else if (unlikely(is_hugetlb_entry_hwpoisoned(entry))) {
_

Patches currently in -mm which might be from linmiaohe@huawei.com are

mm-hugetlb-avoid-corrupting-page-mapping-in-hugetlb_mcopy_atomic_pte.patch
mm-page_alloc-minor-clean-up-for-memmap_init_compound.patch
hugetlb_cgroup-remove-unneeded-nr_pages-0-check.patch
hugetlb_cgroup-hugetlbfs-use-helper-macro-sz_1kmg.patch
hugetlb_cgroup-remove-unneeded-return-value.patch
hugetlb_cgroup-use-helper-macro-numa_no_node.patch
hugetlb_cgroup-use-helper-for_each_hstate-and-hstate_index.patch
mm-hugetlb-fix-incorrect-update-of-max_huge_pages.patch
mm-hugetlb-fix-warn_onkobj-in-sysfs_create_group.patch
mm-hugetlb-fix-missing-call-to-restore_reserve_on_error.patch
mm-hugetlb_vmemmap-add-missing-smp_wmb-before-set_pte_at.patch
mm-hugetlb-fix-sysfs-group-leak-in-hugetlb_unregister_node.patch
mm-hugetlb-make-detecting-shared-pte-more-reliable.patch