[PATCH v4 06/20] mm, hugetlb: return a reserved page to a reserved poolif failed

[Joonsoo Kim]

If we fail with a reserved page, just calling put_page() is not sufficient,
because put_page() invoke free_huge_page() at last step and it doesn't
know whether a page comes from a reserved pool or not. So it doesn't do
anything related to reserved count. This makes reserve count lower
than how we need, because reserve count already decrease in
dequeue_huge_page_vma(). This patch fix this situation.

In this patch, PagePrivate() is used for tracking reservation.
When resereved pages are dequeued from reserved pool, Private flag is
assigned to the hugepage until properly mapped. On page returning process,
if there is a hugepage with Private flag, it is considered as the one
returned in certain error path, so that we should restore one
reserve count back in order to preserve certain user's reserved hugepage.

Using Private flag is safe for the hugepage, because it doesn't use the
LRU mechanism so that there is no other user of this page except us.
Therefore we can use this flag safely.

v4:
- ClearPagePrivate() is added in free_huge_page().

v3:
- commit message is added

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 6c8eec2..2b03666 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -572,6 +572,7 @@ retry_cpuset:
 				if (!vma_has_reserves(vma, chg))
 					break;
 
+				SetPagePrivate(page);
 				h->resv_huge_pages--;
 				break;
 			}
@@ -626,15 +627,22 @@ static void free_huge_page(struct page *page)
 	int nid = page_to_nid(page);
 	struct hugepage_subpool *spool =
 		(struct hugepage_subpool *)page_private(page);
+	bool restore_reserve;
 
 	set_page_private(page, 0);
 	page->mapping = NULL;
 	BUG_ON(page_count(page));
 	BUG_ON(page_mapcount(page));
+	restore_reserve = PagePrivate(page);
+	if (restore_reserve)
+		ClearPagePrivate(page);
 
 	spin_lock(&hugetlb_lock);
 	hugetlb_cgroup_uncharge_page(hstate_index(h),
 				     pages_per_huge_page(h), page);
+	if (restore_reserve)
+		h->resv_huge_pages++;
+
 	if (h->surplus_huge_pages_node[nid] && huge_page_order(h) < MAX_ORDER) {
 		/* remove the page from active list */
 		list_del(&page->lru);
@@ -2616,6 +2624,8 @@ retry_avoidcopy:
 	spin_lock(&mm->page_table_lock);
 	ptep = huge_pte_offset(mm, address & huge_page_mask(h));
 	if (likely(pte_same(huge_ptep_get(ptep), pte))) {
+		ClearPagePrivate(new_page);
+
 		/* Break COW */
 		huge_ptep_clear_flush(vma, address, ptep);
 		set_huge_pte_at(mm, address, ptep,
@@ -2727,6 +2737,7 @@ retry:
 					goto retry;
 				goto out;
 			}
+			ClearPagePrivate(page);
 
 			spin_lock(&inode->i_lock);
 			inode->i_blocks += blocks_per_huge_page(h);
@@ -2773,8 +2784,10 @@ retry:
 	if (!huge_pte_none(huge_ptep_get(ptep)))
 		goto backout;
 
-	if (anon_rmap)
+	if (anon_rmap) {
+		ClearPagePrivate(page);
 		hugepage_add_new_anon_rmap(page, vma, address);
+	}
 	else
 		page_dup_rmap(page);
 	new_pte = make_huge_pte(vma, page, ((vma->vm_flags & VM_WRITE)
-- 
1.7.9.5

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