NTFS: Make ntfs_write_block() not instantiate sparse blocks if they are zero.

Signed-off-by: Anton Altaparmakov <aia21@cantab.net>

2 files changed, 23 insertions, 0 deletions

diff --git a/fs/ntfs/ChangeLog b/fs/ntfs/ChangeLog
index ed8d00150931..b1766642baf8 100644
--- a/fs/ntfs/ChangeLog
+++ b/fs/ntfs/ChangeLog
@@ -77,6 +77,8 @@ ToDo/Notes:
           updating the times in the inode in ntfs_setattr().
         - Fixup handling of sparse, compressed, and encrypted attributes in
           fs/ntfs/inode.c::ntfs_read_locked_{,attr_,index_}inode().
+        - Make ntfs_write_block() not instantiate sparse blocks if they contain
+          only zeroes.
 
 2.1.23 - Implement extension of resident files and make writing safe as well as
          many bug fixes, cleanups, and enhancements...
diff --git a/fs/ntfs/aops.c b/fs/ntfs/aops.c
index 78adad7a988d..f3ad36d8b8c9 100644
--- a/fs/ntfs/aops.c
+++ b/fs/ntfs/aops.c
@@ -670,6 +670,27 @@ lock_retry_remap:
                 }
                 /* It is a hole, need to instantiate it. */
                 if (lcn == LCN_HOLE) {
+                        u8 *kaddr;
+                        unsigned long *bpos, *bend;
+
+                        /* Check if the buffer is zero. */
+                        kaddr = kmap_atomic(page, KM_USER0);
+                        bpos = (unsigned long *)(kaddr + bh_offset(bh));
+                        bend = (unsigned long *)((u8*)bpos + blocksize);
+                        do {
+                                if (unlikely(*bpos))
+                                        break;
+                        } while (likely(++bpos < bend));
+                        kunmap_atomic(kaddr, KM_USER0);
+                        if (bpos == bend) {
+                                /*
+                                 * Buffer is zero and sparse, no need to write
+                                 * it.
+                                 */
+                                bh->b_blocknr = -1;
+                                clear_buffer_dirty(bh);
+                                continue;
+                        }
                         // TODO: Instantiate the hole.
                         // clear_buffer_new(bh);
                         // unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);