Patch "mm/khugepaged.c: convert VM_BUG_ON() to collapse fail" has been added to the 4.14-stable tree

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 4.14-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:
mm-khugepaged.c-convert-vm_bug_on-to-collapse-fail.patch
and it can be found in the queue-4.14 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, 22 Mar 2018 16:17:28 -0700
Subject: mm/khugepaged.c: convert VM_BUG_ON() to collapse fail

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

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.

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

@@ -530,7 +530,12 @@ 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);
 
 		/*

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

queue-4.14/mm-khugepaged.c-convert-vm_bug_on-to-collapse-fail.patch queue-4.14/mm-thp-do-not-wait-for-lock_page-in-deferred_split_scan.patch queue-4.14/hugetlbfs-check-for-pgoff-value-overflow.patch queue-4.14/mm-shmem-do-not-wait-for-lock_page-in-shmem_unused_huge_shrink.patch