Patch "thp: fix corner case of munlock() of PTE-mapped THPs" has been added to the 4.8-stable tree

This is a note to let you know that I've just added the patch titled

thp: fix corner case of munlock() of PTE-mapped THPs

to the 4.8-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
thp-fix-corner-case-of-munlock-of-pte-mapped-thps.patch
and it can be found in the queue-4.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Kirill A. Shutemov" <[email protected]>
Date: Wed, 30 Nov 2016 15:54:05 -0800
Subject: thp: fix corner case of munlock() of PTE-mapped THPs

From: Kirill A. Shutemov <[email protected]>

commit 655548bf6271b212cd1e4c259da9dbe616348d38 upstream.

The following program triggers BUG() in munlock_vma_pages_range():

	// autogenerated by syzkaller (http://github.com/google/syzkaller)
	#include <sys/mman.h>

	int main()
	{
	  mmap((void*)0x20105000ul, 0xc00000ul, 0x2ul, 0x2172ul, -1, 0);
	  mremap((void*)0x201fd000ul, 0x4000ul, 0xc00000ul, 0x3ul, 0x203f0000ul);
	  return 0;
	}

The test-case constructs the situation when munlock_vma_pages_range() finds PTE-mapped THP-head in the middle of page table and, by mistake, skips HPAGE_PMD_NR pages after that.

As result, on the next iteration it hits the middle of PMD-mapped THP and gets upset seeing mlocked tail page.

The solution is only skip HPAGE_PMD_NR pages if the THP was mlocked during munlock_vma_page(). It would guarantee that the page is PMD-mapped as we never mlock PTE-mapeed THPs.

---
 mm/mlock.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

@@ -190,10 +190,13 @@ unsigned int munlock_vma_page(struct pag
 	 */
 	spin_lock_irq(zone_lru_lock(zone));
 
-	nr_pages = hpage_nr_pages(page);
-	if (!TestClearPageMlocked(page))
+	if (!TestClearPageMlocked(page)) {
+		/* Potentially, PTE-mapped THP: do not skip the rest PTEs */
+		nr_pages = 1;
 		goto unlock_out;
+	}
 
+	nr_pages = hpage_nr_pages(page);
 	__mod_zone_page_state(zone, NR_MLOCK, -nr_pages);
 
 	if (__munlock_isolate_lru_page(page, true)) {

Patches currently in stable-queue which might be from [email protected] are

queue-4.8/mm-thp-propagation-of-conditional-compilation-in-khugepaged.c.patch queue-4.8/mm-fix-false-positive-warn_on-in-truncate-invalidate-for-hugetlb.patch queue-4.8/thp-fix-corner-case-of-munlock-of-pte-mapped-thps.patch