[PATCH] xfs: handle EOF correctly in xfs_vm_writepage

[Christoph Hellwig <hch@xxxxxxxxxxxxx>]

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@xxxxxx>

---
 fs/xfs/xfs_aops.c |   19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

Index: xfs/fs/xfs/xfs_aops.c
===================================================================
--- xfs.orig/fs/xfs/xfs_aops.c  2012-06-26 23:20:22.286407388 +0200
+++ xfs/fs/xfs/xfs_aops.c       2012-06-26 23:21:49.323073534 +0200
@@ -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,