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mm/hugetlb: make detecting shared pte more reliable

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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: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Miaohe Lin 2022-08-16 21:05:53 +08:00 committed by Andrew Morton
parent 01088a6036
commit 3aa4ed8040
1 changed files with 8 additions and 13 deletions
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21
mm/hugetlb.c
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@ -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 @@ again:
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);