mm/khugepaged: collapse_shmem() stop if punched or truncated

Huge tmpfs testing showed that although collapse_shmem() recognizes a
concurrently truncated or hole-punched page correctly, its handling of
holes was liable to refill an emptied extent.  Add check to stop that.

Link: http://lkml.kernel.org/r/alpine.LSU.2.11.1811261522040.2275@eggly.anvils
Fixes: f3f0e1d2150b2 ("khugepaged: add support of collapse for tmpfs/shmem pages")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Matthew Wilcox <willy@infradead.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: <stable@vger.kernel.org>	[4.8+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index c13625c1ad5e5e91abf1d45bdbcf388f9b1f2690..2070c316f06ef5909580c85e2bccbf250c9e366c 100644 (file)
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1359,6 +1359,17 @@ static void collapse_shmem(struct mm_struct *mm,
 
                VM_BUG_ON(index != xas.xa_index);
                if (!page) {
+                       /*
+                        * Stop if extent has been truncated or hole-punched,
+                        * and is now completely empty.
+                        */
+                       if (index == start) {
+                               if (!xas_next_entry(&xas, end - 1)) {
+                                       result = SCAN_TRUNCATED;
+                                       break;
+                               }
+                               xas_set(&xas, index);
+                       }
                        if (!shmem_charge(mapping->host, 1)) {
                                result = SCAN_FAIL;
                                break;