[PATCH] 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>
Cc: Miklos Szeredi <mszeredi@suse.cz>
Cc: Badari Pulavarty <pbadari@us.ibm.com>
Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index 578eceafba4a..b2a35ebf071a 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -674,8 +674,16 @@ 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);