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

mm/khugepaged.c: convert VM_BUG_ON() to collapse fail

to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: mm-khugepaged.c-convert-vm_bug_on-to-collapse-fail.patch and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From fece2029a9e65b9a990831afe2a2b83290cbbe26 Mon Sep 17 00:00:00 2001

From: "Kirill A. Shutemov" kirill.shutemov@linux.intel.com Date: Thu, 22 Mar 2018 16:17:28 -0700 Subject: mm/khugepaged.c: convert VM_BUG_ON() to collapse fail

From: Kirill A. Shutemov kirill.shutemov@linux.intel.com

commit fece2029a9e65b9a990831afe2a2b83290cbbe26 upstream.

khugepaged is not yet able to convert PTE-mapped huge pages back to PMD mapped. We do not collapse such pages. See check khugepaged_scan_pmd().

But if between khugepaged_scan_pmd() and __collapse_huge_page_isolate() somebody managed to instantiate THP in the range and then split the PMD back to PTEs we would have a problem -- VM_BUG_ON_PAGE(PageCompound(page)) will get triggered.

It's possible since we drop mmap_sem during collapse to re-take for write.

Replace the VM_BUG_ON() with graceful collapse fail.

Link: http://lkml.kernel.org/r/20180315152353.27989-1-kirill.shutemov@linux.intel.... Fixes: b1caa957ae6d ("khugepaged: ignore pmd tables with THP mapped with ptes") Signed-off-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com Cc: Laura Abbott labbott@redhat.com Cc: Jerome Marchand jmarchan@redhat.com Cc: Vlastimil Babka vbabka@suse.cz Cc: stable@vger.kernel.org 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

--- mm/khugepaged.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -528,7 +528,12 @@ static int __collapse_huge_page_isolate( goto out; }

- VM_BUG_ON_PAGE(PageCompound(page), page); + /* TODO: teach khugepaged to collapse THP mapped with pte */ + if (PageCompound(page)) { + result = SCAN_PAGE_COMPOUND; + goto out; + } + VM_BUG_ON_PAGE(!PageAnon(page), page); VM_BUG_ON_PAGE(!PageSwapBacked(page), page);

Patches currently in stable-queue which might be from kirill.shutemov@linux.intel.com are

queue-4.9/mm-khugepaged.c-convert-vm_bug_on-to-collapse-fail.patch queue-4.9/mm-thp-do-not-wait-for-lock_page-in-deferred_split_scan.patch queue-4.9/mm-shmem-do-not-wait-for-lock_page-in-shmem_unused_huge_shrink.patch