Patch "thp: fix MADV_DONTNEED vs. MADV_FREE race" has been added to the 4.10-stable tree

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

thp: fix MADV_DONTNEED vs. MADV_FREE race

to the 4.10-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-madv_dontneed-vs.-madv_free-race.patch
and it can be found in the queue-4.10 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: Thu, 13 Apr 2017 14:56:26 -0700
Subject: thp: fix MADV_DONTNEED vs. MADV_FREE race

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

commit 58ceeb6bec86d9140f9d91d71a710e963523d063 upstream.

Both MADV_DONTNEED and MADV_FREE handled with down_read(mmap_sem).

It's critical to not clear pmd intermittently while handling MADV_FREE to avoid race with MADV_DONTNEED:

	CPU0:				CPU1:
				madvise_free_huge_pmd()
				 pmdp_huge_get_and_clear_full()

madvise_dontneed()
zap_pmd_range()
pmd_trans_huge(*pmd) == 0 (without ptl)
// skip the pmd

				 set_pmd_at();
				 // pmd is re-established

It results in MADV_DONTNEED skipping the pmd, leaving it not cleared. It violates MADV_DONTNEED interface and can result is userspace misbehaviour.

Basically it's the same race as with numa balancing in change_huge_pmd(), but a bit simpler to mitigate: we don't need to preserve dirty/young flags here due to MADV_FREE functionality.

---
 mm/huge_memory.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

@@ -1393,8 +1393,7 @@ bool madvise_free_huge_pmd(struct mmu_ga
 		deactivate_page(page);
 
 	if (pmd_young(orig_pmd) || pmd_dirty(orig_pmd)) {
-		orig_pmd = pmdp_huge_get_and_clear_full(tlb->mm, addr, pmd,
-			tlb->fullmm);
+		pmdp_invalidate(vma, addr, pmd);
 		orig_pmd = pmd_mkold(orig_pmd);
 		orig_pmd = pmd_mkclean(orig_pmd);
 

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

queue-4.10/thp-fix-madv_dontneed-vs-clear-soft-dirty-race.patch queue-4.10/thp-fix-madv_dontneed-vs.-madv_free-race.patch