[PATCH] thp: fix another corner case of munlock() vs. THPs

[Kirill A. Shutemov]

The following test case triggers BUG() in munlock_vma_pages_range():

	int main(int argc, char *argv[])
	{
		int fd;

		system("mount -t tmpfs -o huge=always none /mnt");
		fd = open("/mnt/test", O_CREAT | O_RDWR);
		ftruncate(fd, 4UL << 20);
		mmap(NULL, 4UL << 20, PROT_READ | PROT_WRITE,
				MAP_SHARED | MAP_FIXED | MAP_LOCKED, fd, 0);
		mmap(NULL, 4096, PROT_READ | PROT_WRITE,
				MAP_SHARED | MAP_LOCKED, fd, 0);
		munlockall();
		return 0;
	}

The second mmap() create PTE-mapping of the first huge page in file. It
makes kernel munlock the page as we never keep PTE-mapped page mlocked.

On munlockall() when we handle vma created by the first mmap(),
munlock_vma_page() returns page_mask == 0, as the page is not mlocked
anymore. On next iteration follow_page_mask() return tail page, but
page_mask is HPAGE_NR_PAGES - 1. It makes us skip to the first tail page
of the next huge page and step on VM_BUG_ON_PAGE(PageMlocked(page)).

The fix is not use the page_mask from follow_page_mask() at all. It has
no use for us.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>    [4.5+]
---
 mm/mlock.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/mm/mlock.c b/mm/mlock.c
index cdbed8aaa426..665ab75b5533 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -441,7 +441,7 @@ void munlock_vma_pages_range(struct vm_area_struct *vma,
 
 	while (start < end) {
 		struct page *page;
-		unsigned int page_mask;
+		unsigned int page_mask = 0;
 		unsigned long page_increm;
 		struct pagevec pvec;
 		struct zone *zone;
@@ -455,8 +455,7 @@ void munlock_vma_pages_range(struct vm_area_struct *vma,
 		 * suits munlock very well (and if somehow an abnormal page
 		 * has sneaked into the range, we won't oops here: great).
 		 */
-		page = follow_page_mask(vma, start, FOLL_GET | FOLL_DUMP,
-				&page_mask);
+		page = follow_page(vma, start, FOLL_GET | FOLL_DUMP);
 
 		if (page && !IS_ERR(page)) {
 			if (PageTransTail(page)) {
@@ -467,8 +466,8 @@ void munlock_vma_pages_range(struct vm_area_struct *vma,
 				/*
 				 * Any THP page found by follow_page_mask() may
 				 * have gotten split before reaching
-				 * munlock_vma_page(), so we need to recompute
-				 * the page_mask here.
+				 * munlock_vma_page(), so we need to compute
+				 * the page_mask here instead.
 				 */
 				page_mask = munlock_vma_page(page);
 				unlock_page(page);
-- 
2.11.0