[RFC v4 PATCH 6/9] mm/hugetlb: alloc_huge_page handle areas hole punched by fallocate

[Mike Kravetz <mike.kravetz@oracle.com>]

Areas hole punched by fallocate will not have entries in the
region/reserve map.  However, shared mappings with min_size subpool
reservations may still have reserved pages.  alloc_huge_page needs
to handle this special case and do the proper accounting.

Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
---
 mm/hugetlb.c | 48 +++++++++++++++++++++++++++---------------------
 1 file changed, 27 insertions(+), 21 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ecbaffe..9c295c9 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -692,19 +692,9 @@ static int vma_has_reserves(struct vm_area_struct *vma, long chg)
 			return 0;
 	}
 
-	if (vma->vm_flags & VM_MAYSHARE) {
-		/*
-		 * We know VM_NORESERVE is not set.  Therefore, there SHOULD
-		 * be a region map for all pages.  The only situation where
-		 * there is no region map is if a hole was punched via
-		 * fallocate.  In this case, there really are no reverves to
-		 * use.  This situation is indicated if chg != 0.
-		 */
-		if (chg)
-			return 0;
-		else
-			return 1;
-	}
+	/* Shared mappings always use reserves */
+	if (vma->vm_flags & VM_MAYSHARE)
+		return 1;
 
 	/*
 	 * Only the process that called mmap() has reserves for
@@ -1601,6 +1591,7 @@ static struct page *alloc_huge_page(struct vm_area_struct *vma,
 	struct hstate *h = hstate_vma(vma);
 	struct page *page;
 	long chg, commit;
+	long gbl_chg;
 	int ret, idx;
 	struct hugetlb_cgroup *h_cg;
 
@@ -1608,24 +1599,39 @@ static struct page *alloc_huge_page(struct vm_area_struct *vma,
 	/*
 	 * Processes that did not create the mapping will have no
 	 * reserves and will not have accounted against subpool
-	 * limit. Check that the subpool limit can be made before
-	 * satisfying the allocation MAP_NORESERVE mappings may also
-	 * need pages and subpool limit allocated allocated if no reserve
-	 * mapping overlaps.
+	 * limit. Check that the subpool limit will not be exceeded
+	 * before performing the allocation.  Allocations for
+	 * MAP_NORESERVE mappings also need to be checked against
+	 * any subpool limit.
+	 *
+	 * NOTE: Shared mappings with holes punched via fallocate
+	 * may still have reservations, even without entries in the
+	 * reserve map as indicated by vma_needs_reservation.  This
+	 * would be the case if hugepage_subpool_get_pages returns
+	 * zero to indicate no changes to the global reservation count
+	 * are necessary.  In this case, pass the output of
+	 * hugepage_subpool_get_pages (zero) to dequeue_huge_page_vma
+	 * so that the page is not counted against the global limit.
+	 * For MAP_NORESERVE mappings always pass the output of
+	 * vma_needs_reservation.  For race detection and error cleanup
+	 * use output of vma_needs_reservation as well.
 	 */
-	chg = vma_needs_reservation(h, vma, addr);
+	chg = gbl_chg = vma_needs_reservation(h, vma, addr);
 	if (chg < 0)
 		return ERR_PTR(-ENOMEM);
-	if (chg || avoid_reserve)
-		if (hugepage_subpool_get_pages(spool, 1) < 0)
+	if (chg || avoid_reserve) {
+		gbl_chg = hugepage_subpool_get_pages(spool, 1);
+		if (gbl_chg < 0)
 			return ERR_PTR(-ENOSPC);
+	}
 
 	ret = hugetlb_cgroup_charge_cgroup(idx, pages_per_huge_page(h), &h_cg);
 	if (ret)
 		goto out_subpool_put;
 
 	spin_lock(&hugetlb_lock);
-	page = dequeue_huge_page_vma(h, vma, addr, avoid_reserve, chg);
+	page = dequeue_huge_page_vma(h, vma, addr, avoid_reserve,
+					avoid_reserve ? chg : gbl_chg);
 	if (!page) {
 		spin_unlock(&hugetlb_lock);
 		page = alloc_buddy_huge_page(h, NUMA_NO_NODE);
-- 
2.1.0

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