From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Subject: thp: fix another corner case of munlock() vs. THPs
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@xxxxxxxxxxxxxxx
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [4.5+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/mlock.c | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)
diff -puN mm/mlock.c~thp-fix-another-corner-case-of-munlock-vs-thps mm/mlock.c
--- a/mm/mlock.c~thp-fix-another-corner-case-of-munlock-vs-thps
+++ a/mm/mlock.c
@@ -442,7 +442,7 @@ void munlock_vma_pages_range(struct vm_a
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;
@@ -456,8 +456,7 @@ void munlock_vma_pages_range(struct vm_a
* 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)) {
@@ -468,8 +467,8 @@ void munlock_vma_pages_range(struct vm_a
/*
* 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);
_