Change on-disk format to support 2^15 uninitialized extents

This change was suggested by Andreas Dilger. 
This patch changes the EXT_MAX_LEN value and extent code which marks/checks
uninitialized extents. With this change it will be possible to have
initialized extents with 2^15 blocks (earlier the max blocks we could have
was 2^15 - 1). This way we can have better extent-to-block alignment.
Now, maximum number of blocks we can have in an initialized extent is 2^15
and in an uninitialized extent is 2^15 - 1.

Signed-off-by: Amit Arora <aarora@in.ibm.com>

1 files changed, 27 insertions, 4 deletions

diff --git a/include/linux/ext4_fs_extents.h b/include/linux/ext4_fs_extents.h
index edf49ec89eac..81406f3655d4 100644
--- a/include/linux/ext4_fs_extents.h
+++ b/include/linux/ext4_fs_extents.h
@@ -141,7 +141,25 @@ typedef int (*ext_prepare_callback)(struct inode *, struct ext4_ext_path *,
 
 #define EXT_MAX_BLOCK   0xffffffff
 
-#define EXT_MAX_LEN     ((1UL << 15) - 1)
+/*
+ * EXT_INIT_MAX_LEN is the maximum number of blocks we can have in an
+ * initialized extent. This is 2^15 and not (2^16 - 1), since we use the
+ * MSB of ee_len field in the extent datastructure to signify if this
+ * particular extent is an initialized extent or an uninitialized (i.e.
+ * preallocated).
+ * EXT_UNINIT_MAX_LEN is the maximum number of blocks we can have in an
+ * uninitialized extent.
+ * If ee_len is <= 0x8000, it is an initialized extent. Otherwise, it is an
+ * uninitialized one. In other words, if MSB of ee_len is set, it is an
+ * uninitialized extent with only one special scenario when ee_len = 0x8000.
+ * In this case we can not have an uninitialized extent of zero length and
+ * thus we make it as a special case of initialized extent with 0x8000 length.
+ * This way we get better extent-to-group alignment for initialized extents.
+ * Hence, the maximum number of blocks we can have in an *initialized*
+ * extent is 2^15 (32768) and in an *uninitialized* extent is 2^15-1 (32767).
+ */
+#define EXT_INIT_MAX_LEN        (1UL << 15)
+#define EXT_UNINIT_MAX_LEN      (EXT_INIT_MAX_LEN - 1)
 
 
 #define EXT_FIRST_EXTENT(__hdr__) \
@@ -190,17 +208,22 @@ ext4_ext_invalidate_cache(struct inode *inode)
 
 static inline void ext4_ext_mark_uninitialized(struct ext4_extent *ext)
 {
-        ext->ee_len |= cpu_to_le16(0x8000);
+        /* We can not have an uninitialized extent of zero length! */
+        BUG_ON((le16_to_cpu(ext->ee_len) & ~EXT_INIT_MAX_LEN) == 0);
+        ext->ee_len |= cpu_to_le16(EXT_INIT_MAX_LEN);
 }
 
 static inline int ext4_ext_is_uninitialized(struct ext4_extent *ext)
 {
-        return (int)(le16_to_cpu((ext)->ee_len) & 0x8000);
+        /* Extent with ee_len of 0x8000 is treated as an initialized extent */
+        return (le16_to_cpu(ext->ee_len) > EXT_INIT_MAX_LEN);
 }
 
 static inline int ext4_ext_get_actual_len(struct ext4_extent *ext)
 {
-        return (int)(le16_to_cpu((ext)->ee_len) & 0x7FFF);
+        return (le16_to_cpu(ext->ee_len) <= EXT_INIT_MAX_LEN ?
+                le16_to_cpu(ext->ee_len) :
+                (le16_to_cpu(ext->ee_len) - EXT_INIT_MAX_LEN));
 }
 
 extern int ext4_extent_tree_init(handle_t *, struct inode *);