[PATCH] huegtlbfs: fix page leak during migration of file pages

[Mike Kravetz]

Files can be created and mapped in an explicitly mounted hugetlbfs
filesystem.  If pages in such files are migrated, the filesystem
usage will not be decremented for the associated pages.  This can
result in mmap or page allocation failures as it appears there are
fewer pages in the filesystem than there should be.

For example, a test program which hole punches, faults and migrates
pages in such a file (1G in size) will eventually fail because it
can not allocate a page.  Reported counts and usage at time of failure:

node0
537	free_hugepages
1024	nr_hugepages
0	surplus_hugepages
node1
1000	free_hugepages
1024	nr_hugepages
0	surplus_hugepages

Filesystem                         Size  Used Avail Use% Mounted on
nodev                              4.0G  4.0G     0 100% /var/opt/hugepool

Note that the filesystem shows 4G of pages used, while actual usage is
511 pages (just under 1G).  Failed trying to allocate page 512.

If a hugetlb page is associated with an explicitly mounted filesystem,
this information in contained in the page_private field.  At migration
time, this information is not preserved.  To fix, simply transfer
page_private from old to new page at migration time if necessary. Also,
migrate_page_states() unconditionally clears page_private and PagePrivate
of the old page.  It is unlikely, but possible that these fields could
be non-NULL and are needed at hugetlb free page time.  So, do not touch
these fields for hugetlb pages.

Cc: <stable@xxxxxxxxxxxxxxx>
Fixes: 290408d4a250 ("hugetlb: hugepage migration core")
Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 fs/hugetlbfs/inode.c | 10 ++++++++++
 mm/migrate.c         | 10 ++++++++--
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 32920a10100e..fb6de1db8806 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -859,6 +859,16 @@ static int hugetlbfs_migrate_page(struct address_space *mapping,
 	rc = migrate_huge_page_move_mapping(mapping, newpage, page);
 	if (rc != MIGRATEPAGE_SUCCESS)
 		return rc;
+
+	/*
+	 * page_private is subpool pointer in hugetlb pages, transfer
+	 * if needed.
+	 */
+	if (page_private(page) && !page_private(newpage)) {
+		set_page_private(newpage, page_private(page));
+		set_page_private(page, 0);
+	}
+
 	if (mode != MIGRATE_SYNC_NO_COPY)
 		migrate_page_copy(newpage, page);
 	else
diff --git a/mm/migrate.c b/mm/migrate.c
index f7e4bfdc13b7..0d9708803553 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -703,8 +703,14 @@ void migrate_page_states(struct page *newpage, struct page *page)
 	 */
 	if (PageSwapCache(page))
 		ClearPageSwapCache(page);
-	ClearPagePrivate(page);
-	set_page_private(page, 0);
+	/*
+	 * Unlikely, but PagePrivate and page_private could potentially
+	 * contain information needed at hugetlb free page time.
+	 */
+	if (!PageHuge(page)) {
+		ClearPagePrivate(page);
+		set_page_private(page, 0);
+	}
 
 	/*
 	 * If any waiters have accumulated on the new page then
-- 
2.17.2