holepunch: fix disconnected pages after second truncate

shmem_truncate_range has its own truncate_inode_pages_range, to free any
pages racily instantiated while it was in progress: a SHMEM_PAGEIN flag
is set when this might have happened.  But holepunching gets no chance
to clear that flag at the start of vmtruncate_range, so it's always set
(unless a truncate came just before), so holepunch almost always does
this second truncate_inode_pages_range.

shmem holepunch has unlikely swap<->file races hereabouts whatever we do
(without a fuller rework than is fit for this release): I was going to
skip the second truncate in the punch_hole case, but Miklos points out
that would make holepunch correctness more vulnerable to swapoff.  So
keep the second truncate, but follow it by an unmap_mapping_range to
eliminate the disconnected pages (freed from pagecache while still
mapped in userspace) that it might have left behind.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

mm/shmem.c

@@ -669,8 +669,16 @@ done2:
 		 * generic_delete_inode did it, before we lowered next_index.
 		 * Also, though shmem_getpage checks i_size before adding to
 		 * cache, no recheck after: so fix the narrow window there too.
+		 *
+		 * Recalling truncate_inode_pages_range and unmap_mapping_range
+		 * every time for punch_hole (which never got a chance to clear
+		 * SHMEM_PAGEIN at the start of vmtruncate_range) is expensive,
+		 * yet hardly ever necessary: try to optimize them out later.
 		 */
 		truncate_inode_pages_range(inode->i_mapping, start, end);
+		if (punch_hole)
+			unmap_mapping_range(inode->i_mapping, start,
+							end - start, 1);
 	}
 
 	spin_lock(&info->lock);