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

commit 6ebb4a1b848fe75323135f93e72c78f8780fd268 upstream.

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.

Link: http://lkml.kernel.org/r/20170302150252.34120-1-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files 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);