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

commit 701270fa193aadf00bdcf607738f64997275d4c7 upstream.

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 1df37ee..62de241 100644 (file)
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1352,6 +1352,16 @@ static void collapse_shmem(struct mm_struct *mm,
                int n = min(iter.index, end) - index;
 
                /*
+                * Stop if extent has been hole-punched, and is now completely
+                * empty (the more obvious i_size_read() check would take an
+                * irq-unsafe seqlock on 32-bit).
+                */
+               if (n >= HPAGE_PMD_NR) {
+                       result = SCAN_TRUNCATED;
+                       goto tree_locked;
+               }
+
+               /*
                 * Handle holes in the radix tree: charge it from shmem and
                 * insert relevant subpage of new_page into the radix-tree.
                 */
@@ -1462,6 +1472,11 @@ out_unlock:
        if (result == SCAN_SUCCEED && index < end) {
                int n = end - index;
 
+               /* Stop if extent has been truncated, and is now empty */
+               if (n >= HPAGE_PMD_NR) {
+                       result = SCAN_TRUNCATED;
+                       goto tree_locked;
+               }
                if (!shmem_charge(mapping->host, n)) {
                        result = SCAN_FAIL;
                        goto tree_locked;