[PATCH 2/3] hugetlb: Close a difficult to trigger reservation race

[Adam Litke <agl@us.ibm.com>]

If the following occurs in threads A and B:
 A) Allocates some surplus pages to satisfy a reservation
 B) Frees some huge pages
 A) A notices the extra free pages and drops hugetlb_lock to free some of
    its surplus pages back to the buddy allocator.
 B) Allocates some huge pages
 A) Reacquires hugetlb_lock and returns from gather_surplus_huge_pages()

This series of events can result in a "short" reservation and subsequent
application failure.

Avoid this by commiting the reservation after pages have been allocated but
before dropping the lock to free excess pages.  For parity, release the
reservation in return_unused_surplus_pages().

Thanks to Andy Whitcroft for discovering this.

This does make the functions gather_surplus_huge_pages() and
return_unused_surplus_pages() responsible for a bit more than their names
imply.  Does anyone support function name changes to
create_reservation() and destroy_reservation() respectively?

Signed-off-by: Adam Litke <agl@us.ibm.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Andy Whitcroft <apw@shadowen.org>
---

 mm/hugetlb.c |   17 +++++++++++++----
 1 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 026e5ee..8296431 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -300,8 +300,10 @@ static int gather_surplus_pages(int delta)
 	int needed, allocated;
 
 	needed = (resv_huge_pages + delta) - free_huge_pages;
-	if (needed <= 0)
+	if (needed <= 0) {
+		resv_huge_pages += delta;
 		return 0;
+	}
 
 	allocated = 0;
 	INIT_LIST_HEAD(&surplus_list);
@@ -339,9 +341,12 @@ retry:
 	 * The surplus_list now contains _at_least_ the number of extra pages
 	 * needed to accomodate the reservation.  Add the appropriate number
 	 * of pages to the hugetlb pool and free the extras back to the buddy
-	 * allocator.
+	 * allocator.  Commit the entire reservation here to prevent another
+	 * process from stealing the pages as they are added to the pool but
+	 * before they are reserved.
 	 */
 	needed += allocated;
+	resv_huge_pages += delta;
 	ret = 0;
 free:
 	list_for_each_entry_safe(page, tmp, &surplus_list, lru) {
@@ -376,6 +381,9 @@ static void return_unused_surplus_pages(unsigned long unused_resv_pages)
 	struct page *page;
 	unsigned long nr_pages;
 
+	/* Uncommit the reservation */
+	resv_huge_pages -= unused_resv_pages;
+
 	nr_pages = min(unused_resv_pages, surplus_huge_pages);
 
 	while (nr_pages) {
@@ -1197,12 +1205,13 @@ static int hugetlb_acct_memory(long delta)
 		if (gather_surplus_pages(delta) < 0)
 			goto out;
 
-		if (delta > cpuset_mems_nr(free_huge_pages_node))
+		if (delta > cpuset_mems_nr(free_huge_pages_node)) {
+			return_unused_surplus_pages(delta);
 			goto out;
+		}
 	}
 
 	ret = 0;
-	resv_huge_pages += delta;
 	if (delta < 0)
 		return_unused_surplus_pages((unsigned long) -delta);
 

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