Re: [PATCH v2 2/5] mm: Clear vmf->pte after pte_unmap_same() returns

Liam Howlett
2021-09-08T01:12:22Z

* Peter Xu <peterx@redhat.com> [210902 16:17]:
> pte_unmap_same() will always unmap the pte pointer. After the unmap, vmf->pte
> will not be valid any more, we should clear it.
>
> It was safe only because no one is accessing vmf->pte after pte_unmap_same()
> returns, since the only caller of pte_unmap_same() (so far) is do_swap_page(),
> where vmf->pte will in most cases be overwritten very soon.
>
> Directly pass in vmf into pte_unmap_same() and then we can also avoid the long
> parameter list too, which should be a nice cleanup.
>
> Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> mm/memory.c | 12 ++++++------
> 1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 25fc46e87214..7b095f07c4ef 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2724,19 +2724,19 @@ EXPORT_SYMBOL_GPL(apply_to_existing_page_range);
> * proceeding (but do_wp_page is only called after already making such a check;
> * and do_anonymous_page can safely check later on).
> */
> -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
> - pte_t *page_table, pte_t orig_pte)
> +static inline int pte_unmap_same(struct vm_fault *vmf)
> {
> int same = 1;
> #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPTION)
> if (sizeof(pte_t) > sizeof(unsigned long)) {
> - spinlock_t *ptl = pte_lockptr(mm, pmd);
> + spinlock_t *ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
> spin_lock(ptl);
> - same = pte_same(*page_table, orig_pte);
> + same = pte_same(*vmf->pte, vmf->orig_pte);
> spin_unlock(ptl);
> }
> #endif
> - pte_unmap(page_table);
> + pte_unmap(vmf->pte);
> + vmf->pte = NULL;
> return same;
> }
>
> @@ -3487,7 +3487,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> vm_fault_t ret = 0;
> void *shadow = NULL;
>
> - if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
> + if (!pte_unmap_same(vmf))
> goto out;
>
> entry = pte_to_swp_entry(vmf->orig_pte);
> --
> 2.31.1
>

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>