Re: [PATCH 4/4] hugetlb: Do early cow when page pinned on src mm

Mike Kravetz
2021-02-03T22:06:18Z

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;
> }
>