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

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/[email protected]
Fixes: b1caa957ae6d ("khugepaged: ignore pmd tables with THP mapped with ptes")
Signed-off-by: Kirill A. Shutemov <[email protected]>
Cc: Laura Abbott <[email protected]>
Cc: Jerome Marchand <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index b7e2268dfc9a15c64eced7bfb857e9a32fc128c1..c15da1ea7e639bc0ab56bc747a4f7110c38f7187 100644 (file)
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -530,7 +530,12 @@ 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);
 
                /*