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    SubjectRe: [PATCH 4/4] hugetlb: Do early cow when page pinned on src mm
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    Date
    On 2/3/21 1:08 PM, Peter Xu wrote:
    > This is the last missing piece of the COW-during-fork effort when there're
    > pinned pages found. One can reference 70e806e4e645 ("mm: Do early cow for
    > pinned pages during fork() for ptes", 2020-09-27) for more information, since
    > we do similar things here rather than pte this time, but just for hugetlb.
    >
    > Signed-off-by: Peter Xu <peterx@redhat.com>
    > ---
    > mm/hugetlb.c | 76 ++++++++++++++++++++++++++++++++++++++++++++++++----
    > 1 file changed, 71 insertions(+), 5 deletions(-)
    >
    > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
    > index 9e6ea96bf33b..931bf1a81c16 100644
    > --- a/mm/hugetlb.c
    > +++ b/mm/hugetlb.c
    > @@ -3734,11 +3734,27 @@ static bool is_hugetlb_entry_hwpoisoned(pte_t pte)
    > return false;
    > }
    >
    > +static void
    > +hugetlb_copy_page(struct vm_area_struct *vma, pte_t *ptep, unsigned long addr,
    > + struct page *old_page, struct page *new_page)
    > +{
    > + struct hstate *h = hstate_vma(vma);
    > + unsigned int psize = pages_per_huge_page(h);
    > +
    > + copy_user_huge_page(new_page, old_page, addr, vma, psize);
    > + __SetPageUptodate(new_page);
    > + ClearPagePrivate(new_page);
    > + set_page_huge_active(new_page);
    > + set_huge_pte_at(vma->vm_mm, addr, ptep, make_huge_pte(vma, new_page, 1));
    > + hugepage_add_new_anon_rmap(new_page, vma, addr);
    > + hugetlb_count_add(psize, vma->vm_mm);
    > +}
    > +
    > int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
    > struct vm_area_struct *vma)
    > {
    > pte_t *src_pte, *dst_pte, entry, dst_entry;
    > - struct page *ptepage;
    > + struct page *ptepage, *prealloc = NULL;
    > unsigned long addr;
    > int cow;
    > struct hstate *h = hstate_vma(vma);
    > @@ -3787,7 +3803,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
    > dst_entry = huge_ptep_get(dst_pte);
    > if ((dst_pte == src_pte) || !huge_pte_none(dst_entry))
    > continue;
    > -
    > +again:
    > dst_ptl = huge_pte_lock(h, dst, dst_pte);
    > src_ptl = huge_pte_lockptr(h, src, src_pte);
    > spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
    > @@ -3816,6 +3832,54 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
    > }
    > set_huge_swap_pte_at(dst, addr, dst_pte, entry, sz);
    > } else {
    > + entry = huge_ptep_get(src_pte);
    > + ptepage = pte_page(entry);
    > + get_page(ptepage);
    > +
    > + if (unlikely(page_needs_cow_for_dma(vma, ptepage))) {
    > + /* This is very possibly a pinned huge page */
    > + if (!prealloc) {
    > + /*
    > + * Preallocate the huge page without
    > + * tons of locks since we could sleep.
    > + * Note: we can't use any reservation
    > + * because the page will be exclusively
    > + * owned by the child later.
    > + */
    > + put_page(ptepage);
    > + spin_unlock(src_ptl);
    > + spin_unlock(dst_ptl);
    > + prealloc = alloc_huge_page(vma, addr, 0);

    One quick question:

    The comment says we can't use any reservation, and I agree. However, the
    alloc_huge_page call has 0 as the avoid_reserve argument. Shouldn't that
    be !0 to avoid reserves?

    --
    Mike Kravetz

    > + if (!prealloc) {
    > + /*
    > + * hugetlb_cow() seems to be
    > + * more careful here than us.
    > + * However for fork() we could
    > + * be strict not only because
    > + * no one should be referencing
    > + * the child mm yet, but also
    > + * if resources are rare we'd
    > + * better simply fail the
    > + * fork() even earlier.
    > + */
    > + ret = -ENOMEM;
    > + break;
    > + }
    > + goto again;
    > + }
    > + /*
    > + * We have page preallocated so that we can do
    > + * the copy right now.
    > + */
    > + hugetlb_copy_page(vma, dst_pte, addr, ptepage,
    > + prealloc);
    > + put_page(ptepage);
    > + spin_unlock(src_ptl);
    > + spin_unlock(dst_ptl);
    > + prealloc = NULL;
    > + continue;
    > + }
    > +
    > if (cow) {
    > /*
    > * No need to notify as we are downgrading page
    > @@ -3826,9 +3890,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
    > */
    > huge_ptep_set_wrprotect(src, addr, src_pte);
    > }
    > - entry = huge_ptep_get(src_pte);
    > - ptepage = pte_page(entry);
    > - get_page(ptepage);
    > +
    > page_dup_rmap(ptepage, true);
    > set_huge_pte_at(dst, addr, dst_pte, entry);
    > hugetlb_count_add(pages_per_huge_page(h), dst);
    > @@ -3842,6 +3904,10 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
    > else
    > i_mmap_unlock_read(mapping);
    >
    > + /* Free the preallocated page if not used at last */
    > + if (prealloc)
    > + put_page(prealloc);
    > +
    > return ret;
    > }
    >

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