Re: [PATCH 02/23] mm: Clear vmf->pte after pte_unmap_same() returns

Miaohe Lin
2021-03-23T02:35:28Z

Hi:
On 2021/3/23 8:48, Peter Xu wrote:
> 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.
>
> pte_unmap_same() will be used in other places in follow up patches, so that
> vmf->pte will not always be re-written. This patch enables us to call
> functions like finish_fault() because that'll conditionally unmap the pte by
> checking vmf->pte first. Or, alloc_set_pte() will make sure to allocate a new
> pte even after calling pte_unmap_same().
>
> Since we'll need to modify vmf->pte, directly pass in vmf into pte_unmap_same()
> and then we can also avoid the long parameter list.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>

Good cleanup! Thanks.
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>

> ---
> mm/memory.c | 13 +++++++------
> 1 file changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index a458a595331f..d534eba85756 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2607,19 +2607,20 @@ 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);
> + /* After unmap of pte, the pointer is invalid now - clear it. */
> + vmf->pte = NULL;
> return same;
> }
>
> @@ -3308,7 +3309,7 @@ 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);
>