xfs: handle EOF correctly in xfs_vm_writepage

We need to zero out part of a page which beyond EOF before setting uptodate,
otherwise, mapread or write will see non-zero data beyond EOF.

Based on the code in fs/buffer.c and the following ext4 commit:

  ext4: handle EOF correctly in ext4_bio_write_page()

And yes, I wish we had a good test case for it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

1 files changed, 17 insertions, 2 deletions

diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 84e372596d56..91d77ac51bba 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -927,11 +927,26 @@ xfs_vm_writepage(
         end_index = offset >> PAGE_CACHE_SHIFT;
         last_index = (offset - 1) >> PAGE_CACHE_SHIFT;
         if (page->index >= end_index) {
-                if ((page->index >= end_index + 1) ||
-                    !(i_size_read(inode) & (PAGE_CACHE_SIZE - 1))) {
+                unsigned offset_into_page = offset & (PAGE_CACHE_SIZE - 1);
+
+                /*
+                 * Just skip the page if it is fully outside i_size, e.g. due
+                 * to a truncate operation that is in progress.
+                 */
+                if (page->index >= end_index + 1 || offset_into_page == 0) {
                         unlock_page(page);
                         return 0;
                 }
+
+                /*
+                 * The page straddles i_size.  It must be zeroed out on each
+                 * and every writepage invocation because it may be mmapped.
+                 * "A file is mapped in multiples of the page size.  For a file
+                 * that is not a multiple of the  page size, the remaining
+                 * memory is zeroed when mapped, and writes to that region are
+                 * not written out to the file."
+                 */
+                zero_user_segment(page, offset_into_page, PAGE_CACHE_SIZE);
         }
 
         end_offset = min_t(unsigned long long,