On 2023/5/22 12:58, Hugh Dickins wrote:
> map_pte() use pte_offset_map_nolock(), to make sure of the ptl belonging
> to pte, even if pmd entry is then changed racily: page_vma_mapped_walk()
> use that instead of getting pte_lockptr() later, or restart if map_pte()
> found no page table.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
> mm/page_vma_mapped.c | 28 ++++++++++++++++++++++------
> 1 file changed, 22 insertions(+), 6 deletions(-)
>
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index 947dc7491815..2af734274073 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -13,16 +13,28 @@ static inline bool not_found(struct page_vma_mapped_walk *pvmw)
> return false;
> }
>
> -static bool map_pte(struct page_vma_mapped_walk *pvmw)
> +static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
> {
> if (pvmw->flags & PVMW_SYNC) {
> /* Use the stricter lookup */
> pvmw->pte = pte_offset_map_lock(pvmw->vma->vm_mm, pvmw->pmd,
> pvmw->address, &pvmw->ptl);
> - return true;
> + *ptlp = pvmw->ptl;
> + return !!pvmw->pte;
> }
>
> - pvmw->pte = pte_offset_map(pvmw->pmd, pvmw->address);
> + /*
> + * It is important to return the ptl corresponding to pte,
> + * in case *pvmw->pmd changes underneath us; so we need to
> + * return it even when choosing not to lock, in case caller
> + * proceeds to loop over next ptes, and finds a match later.
> + * Though, in most cases, page lock already protects this.
> + */
> + pvmw->pte = pte_offset_map_nolock(pvmw->vma->vm_mm, pvmw->pmd,
> + pvmw->address, ptlp);
> + if (!pvmw->pte)
> + return false;
> +
> if (pvmw->flags & PVMW_MIGRATION) {
> if (!is_swap_pte(*pvmw->pte))
> return false;
> @@ -51,7 +63,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw)
> } else if (!pte_present(*pvmw->pte)) {
> return false;
> }
> - pvmw->ptl = pte_lockptr(pvmw->vma->vm_mm, pvmw->pmd);
> + pvmw->ptl = *ptlp;
> spin_lock(pvmw->ptl);
> return true;
> }
> @@ -156,6 +168,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> struct vm_area_struct *vma = pvmw->vma;
> struct mm_struct *mm = vma->vm_mm;
> unsigned long end;
> + spinlock_t *ptl;
> pgd_t *pgd;
> p4d_t *p4d;
> pud_t *pud;
> @@ -257,8 +270,11 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> step_forward(pvmw, PMD_SIZE);
> continue;
> }
> - if (!map_pte(pvmw))
> + if (!map_pte(pvmw, &ptl)) {
> + if (!pvmw->pte)
> + goto restart;
Could pvmw->pmd be changed? Otherwise, how about just jumping to the
retry label below?
@@ -205,6 +205,8 @@ bool page_vma_mapped_walk(struct
page_vma_mapped_walk *pvmw)
}
pvmw->pmd = pmd_offset(pud, pvmw->address);
+
+retry:
/*
* Make sure the pmd value isn't cached in a register
by the
* compiler and used as a stale value after we've
observed a
> goto next_pte;
> + }
> this_pte:
> if (check_pte(pvmw))
> return true;
> @@ -281,7 +297,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> } while (pte_none(*pvmw->pte));
>
> if (!pvmw->ptl) {
> - pvmw->ptl = pte_lockptr(mm, pvmw->pmd);
> + pvmw->ptl = ptl;
> spin_lock(pvmw->ptl);
> }
> goto this_pte;
--
Thanks,
Qi