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

Peter Xu
2021-02-03T21:11:12Z

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);
+ 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;
}

--
2.26.2