Messages in this thread | ![/](/images/icornerl.gif) | | Date | Tue, 20 Jun 2023 17:22:02 +0200 | From | David Hildenbrand <> | Subject | Re: [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
| ![\](/images/icornerr.gif) |