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

[Peter Xu <peterx@redhat.com>]

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