[PATCH v3 03/20] mm, hugetlb: fix subpool accounting handling

[Joonsoo Kim]

There is a case that we attempt to allocate a hugepage with chg = 0 and
avoid_reserve = 1. Although chg = 0 means that it has a reserved hugepage,
we wouldn't use it, since avoid_reserve = 1 represents that we don't want
to allocate a hugepage from a reserved pool. This happens when the parent
process that created a MAP_PRIVATE mapping is about to perform a COW due to
a shared page count and it attempt to satisfy the allocation without using
the existing reserves.

In this case, we would not dequeue a reserved hugepage and, instead, try
to allocate a new hugepage. Therefore, we should check subpool counter
for a new hugepage. This patch implement it.

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
---
Replenishing commit message and adding reviewed-by tag.

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 12b6581..ea1ae0a 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1144,13 +1144,14 @@ static struct page *alloc_huge_page(struct vm_area_struct *vma,
 	chg = vma_needs_reservation(h, vma, addr);
 	if (chg < 0)
 		return ERR_PTR(-ENOMEM);
-	if (chg)
-		if (hugepage_subpool_get_pages(spool, chg))
+	if (chg || avoid_reserve)
+		if (hugepage_subpool_get_pages(spool, 1))
 			return ERR_PTR(-ENOSPC);
 
 	ret = hugetlb_cgroup_charge_cgroup(idx, pages_per_huge_page(h), &h_cg);
 	if (ret) {
-		hugepage_subpool_put_pages(spool, chg);
+		if (chg || avoid_reserve)
+			hugepage_subpool_put_pages(spool, 1);
 		return ERR_PTR(-ENOSPC);
 	}
 	spin_lock(&hugetlb_lock);
@@ -1162,7 +1163,8 @@ static struct page *alloc_huge_page(struct vm_area_struct *vma,
 			hugetlb_cgroup_uncharge_cgroup(idx,
 						       pages_per_huge_page(h),
 						       h_cg);
-			hugepage_subpool_put_pages(spool, chg);
+			if (chg || avoid_reserve)
+				hugepage_subpool_put_pages(spool, 1);
 			return ERR_PTR(-ENOSPC);
 		}
 		spin_lock(&hugetlb_lock);
-- 
1.7.9.5

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