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

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.com
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>

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 5d7c006373d30580f97d336c1cbb60edc7927735..898eb26f5dc862ec52f4bd570065db6cfe4635d8 100644 (file)
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -528,7 +528,12 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
                        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);