ext4: return hole from ext4_map_blocks()

Currently, ext4_map_blocks() just returns 0 when it finds a hole and
allocation is not requested. However we have all the information
available to tell how large the hole actually is and there are callers
of ext4_map_blocks() which would save some block-by-block hole iteration
if they knew this information. So fill in struct ext4_map_blocks even
for holes with the information we have. We keep returning 0 for holes to
maintain backward compatibility of the function.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

3 files changed, 36 insertions, 9 deletions

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 7858fc1d2501..2730039d4742 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -4374,13 +4374,20 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
         if ((flags & EXT4_GET_BLOCKS_CREATE) == 0) {
                 ext4_lblk_t hole_start, hole_len;
 
+                hole_start = map->m_lblk;
+                hole_len = ext4_ext_determine_hole(inode, path, &hole_start);
                 /*
                  * put just found gap into cache to speed up
                  * subsequent requests
                  */
-                hole_start = map->m_lblk;
-                hole_len = ext4_ext_determine_hole(inode, path, &hole_start);
                 ext4_ext_put_gap_in_cache(inode, hole_start, hole_len);
+
+                /* Update hole_len to reflect hole size after map->m_lblk */
+                if (hole_start != map->m_lblk)
+                        hole_len -= map->m_lblk - hole_start;
+                map->m_pblk = 0;
+                map->m_len = min_t(unsigned int, map->m_len, hole_len);
+
                 goto out2;
         }
 
diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
index 1655ff195e0e..3027fa681de5 100644
--- a/fs/ext4/indirect.c
+++ b/fs/ext4/indirect.c
@@ -555,8 +555,23 @@ int ext4_ind_map_blocks(handle_t *handle, struct inode *inode,
                 goto got_it;
         }
 
-        /* Next simple case - plain lookup or failed read of indirect block */
-        if ((flags & EXT4_GET_BLOCKS_CREATE) == 0 || err == -EIO)
+        /* Next simple case - plain lookup failed */
+        if ((flags & EXT4_GET_BLOCKS_CREATE) == 0) {
+                unsigned epb = inode->i_sb->s_blocksize / sizeof(u32);
+                int i;
+
+                /* Count number blocks in a subtree under 'partial' */
+                count = 1;
+                for (i = 0; partial + i != chain + depth - 1; i++)
+                        count *= epb;
+                /* Fill in size of a hole we found */
+                map->m_pblk = 0;
+                map->m_len = min_t(unsigned int, map->m_len, count);
+                goto cleanup;
+        }
+
+        /* Failed read of indirect block */
+        if (err == -EIO)
                 goto cleanup;
 
         /*
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 971892d7c213..1a156ec043a0 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -455,13 +455,13 @@ static void ext4_map_blocks_es_recheck(handle_t *handle,
  * Otherwise, call with ext4_ind_map_blocks() to handle indirect mapping
  * based files
  *
- * On success, it returns the number of blocks being mapped or allocated.
- * if create==0 and the blocks are pre-allocated and unwritten block,
- * the result buffer head is unmapped. If the create ==1, it will make sure
- * the buffer head is mapped.
+ * On success, it returns the number of blocks being mapped or allocated.  if
+ * create==0 and the blocks are pre-allocated and unwritten, the resulting @map
+ * is marked as unwritten. If the create == 1, it will mark @map as mapped.
  *
  * It returns 0 if plain look up failed (blocks have not been allocated), in
- * that case, buffer head is unmapped
+ * that case, @map is returned as unmapped but we still do fill map->m_len to
+ * indicate the length of a hole starting at map->m_lblk.
  *
  * It returns the error in case of allocation failure.
  */
@@ -504,6 +504,11 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
                                 retval = map->m_len;
                         map->m_len = retval;
                 } else if (ext4_es_is_delayed(&es) || ext4_es_is_hole(&es)) {
+                        map->m_pblk = 0;
+                        retval = es.es_len - (map->m_lblk - es.es_lblk);
+                        if (retval > map->m_len)
+                                retval = map->m_len;
+                        map->m_len = retval;
                         retval = 0;
                 } else {
                         BUG_ON(1);