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SubjectRe: [PATCH v2 2/8] mm/hugetlb: Prepare hugetlb_follow_page_mask() for FOLL_PIN
On 20.06.23 01:10, Peter Xu wrote:
> follow_page() doesn't use FOLL_PIN, meanwhile hugetlb seems to not be the
> target of FOLL_WRITE either. However add the checks.
>
> Namely, either the need to CoW due to missing write bit, or proper CoR on

s/CoR/unsharing/

> !AnonExclusive pages over R/O pins to reject the follow page. That brings
> this function closer to follow_hugetlb_page().
>
> So we don't care before, and also for now. But we'll care if we switch
> over slow-gup to use hugetlb_follow_page_mask(). We'll also care when to
> return -EMLINK properly, as that's the gup internal api to mean "we should
> do CoR". Not really needed for follow page path, though.

"we should unshare".

>
> When at it, switching the try_grab_page() to use WARN_ON_ONCE(), to be
> clear that it just should never fail.
>
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> mm/hugetlb.c | 24 +++++++++++++++---------
> 1 file changed, 15 insertions(+), 9 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index f75f5e78ff0b..9a6918c4250a 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6463,13 +6463,6 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> spinlock_t *ptl;
> pte_t *pte, entry;
>
> - /*
> - * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
> - * follow_hugetlb_page().
> - */
> - if (WARN_ON_ONCE(flags & FOLL_PIN))
> - return NULL;
> -
> hugetlb_vma_lock_read(vma);
> pte = hugetlb_walk(vma, haddr, huge_page_size(h));
> if (!pte)
> @@ -6478,8 +6471,21 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> ptl = huge_pte_lock(h, mm, pte);
> entry = huge_ptep_get(pte);
> if (pte_present(entry)) {
> - page = pte_page(entry) +
> - ((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
> + page = pte_page(entry);
> +
> + if (gup_must_unshare(vma, flags, page)) {

All other callers (like follow_page_pte(), including
__follow_hugetlb_must_fault())

(a) check for write permissions first.

(b) check for gup_must_unshare() only if !pte_write(entry)

I'd vote to keep these checks as similar as possible to the other GUP code.

> + /* Tell the caller to do Copy-On-Read */

"Tell the caller to unshare".

> + page = ERR_PTR(-EMLINK);
> + goto out;
> + }
> +
> + if ((flags & FOLL_WRITE) && !pte_write(entry)) {
> + page = NULL;
> + goto out;
> + }


I'm confused about pte_write() vs. huge_pte_write(), and I don't know
what's right or wrong here.

--
Cheers,

David / dhildenb

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