Messages in this thread | | | Date | Mon, 26 Jun 2023 10:06:24 +0200 | From | David Hildenbrand <> | Subject | Re: [PATCH v3 2/8] mm/hugetlb: Prepare hugetlb_follow_page_mask() for FOLL_PIN |
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On 23.06.23 16:29, 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 > unsharing on !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 > unshare". Not really needed for follow page path, though. > > When at it, switching the try_grab_page() to use WARN_ON_ONCE(), to be > clear that it just should never fail. When error happens, instead of > setting page==NULL, capture the errno instead. > > Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com> > Signed-off-by: Peter Xu <peterx@redhat.com> > --- > mm/hugetlb.c | 31 ++++++++++++++++++++----------- > 1 file changed, 20 insertions(+), 11 deletions(-) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index f75f5e78ff0b..27367edf5c72 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -6462,13 +6462,7 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma, > struct page *page = NULL; > 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; > + int ret; > > hugetlb_vma_lock_read(vma); > pte = hugetlb_walk(vma, haddr, huge_page_size(h)); > @@ -6478,8 +6472,21 @@ 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 ((flags & FOLL_WRITE) && !huge_pte_write(entry)) { > + page = NULL; > + goto out; > + } > + > + if (gup_must_unshare(vma, flags, page)) { > + /* Tell the caller to do unsharing */ > + page = ERR_PTR(-EMLINK); > + goto out; > + }
No need to check if the page is writable (like all other callers and as gup_must_unshare() documents -- "for which pages that are write-protected in the page table")
if (!huge_pte_write(entry) && gup_must_unshare(vma, flags, page)) {
With that
Reviewed-by: David Hildenbrand <david@redhat.com>
-- Cheers,
David / dhildenb
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