Commit 3aa4ed80 authored by Miaohe Lin's avatar Miaohe Lin Committed by Andrew Morton
Browse files

mm/hugetlb: make detecting shared pte more reliable 

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.

[lukas.bulwahn@gmail.com: remove unused local variable dst_entry in copy_hugetlb_page_range()]
  Link: https://lkml.kernel.org/r/20220822082525.26071-1-lukas.bulwahn@gmail.com
Link: https://lkml.kernel.org/r/20220816130553.31406-7-linmiaohe@huawei.com


Signed-off-by: default avatarMiaohe Lin <linmiaohe@huawei.com>
Signed-off-by: default avatarLukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: default avatarMike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 01088a60

mm/hugetlb.c

+8 −13
Original line number Diff line number Diff line
@@ -4750,7 +4750,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, 
			    struct vm_area_struct *dst_vma,

			    struct vm_area_struct *src_vma)

{

	pte_t *src_pte, *dst_pte, entry, dst_entry;

	pte_t *src_pte, *dst_pte, entry;

	struct page *ptepage;

	unsigned long addr;

	bool cow = is_cow_mapping(src_vma->vm_flags);
@@ -4795,15 +4795,13 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, 


		/*

		 * 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;

		}
@@ -4812,13 +4810,10 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, 
		src_ptl = huge_pte_lockptr(h, src, src_pte);

		spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);

		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))) {
@@ -4897,7 +4892,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, 
					restore_reserve_on_error(h, dst_vma, addr,

								new);

					put_page(new);

					/* dst_entry won't change as in child */

					/* huge_ptep of dst_pte won't change as in child */

					goto again;

				}

				hugetlb_install_page(dst_vma, dst_pte, addr, new);