[PATCH] ext4: uninitialised extent handling

Make it possible to add file preallocation support in future as an RO_COMPAT
feature by recognizing uninitialized extents as holes and limiting extent
length to keep the top bit of ee_len free for marking uninitialized extents.

Signed-off-by: Suparna Bhattacharya <suparna@in.ibm.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 18 insertions, 0 deletions

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 4a13b56e1540..32526061a17d 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -1082,6 +1082,13 @@ ext4_can_extents_be_merged(struct inode *inode, struct ext4_extent *ex1,
             != le32_to_cpu(ex2->ee_block))
                 return 0;
 
+        /*
+         * To allow future support for preallocated extents to be added
+         * as an RO_COMPAT feature, refuse to merge to extents if
+         * can result in the top bit of ee_len being set
+         */
+        if (le16_to_cpu(ex1->ee_len) + le16_to_cpu(ex2->ee_len) > EXT_MAX_LEN)
+                return 0;
 #ifdef AGRESSIVE_TEST
         if (le16_to_cpu(ex1->ee_len) >= 4)
                 return 0;
@@ -1944,6 +1951,15 @@ int ext4_ext_get_blocks(handle_t *handle, struct inode *inode,
                 unsigned long ee_block = le32_to_cpu(ex->ee_block);
                 ext4_fsblk_t ee_start = ext_pblock(ex);
                 unsigned short ee_len  = le16_to_cpu(ex->ee_len);
+
+                /*
+                 * Allow future support for preallocated extents to be added
+                 * as an RO_COMPAT feature:
+                 * Uninitialized extents are treated as holes, except that
+                 * we avoid (fail) allocating new blocks during a write.
+                 */
+                if (ee_len > EXT_MAX_LEN)
+                        goto out2;
                 /* if found exent covers block, simple return it */
                 if (iblock >= ee_block && iblock < ee_block + ee_len) {
                         newblock = iblock - ee_block + ee_start;
diff --git a/include/linux/ext4_fs_extents.h b/include/linux/ext4_fs_extents.h
index facc76684d4f..0eba0acf6ba6 100644
--- a/include/linux/ext4_fs_extents.h
+++ b/include/linux/ext4_fs_extents.h
@@ -141,6 +141,8 @@ typedef int (*ext_prepare_callback)(struct inode *, struct ext4_ext_path *,
 
 #define EXT_MAX_BLOCK   0xffffffff
 
+#define EXT_MAX_LEN     ((1UL << 15) - 1)
+
 
 #define EXT_FIRST_EXTENT(__hdr__) \
         ((struct ext4_extent *) (((char *) (__hdr__)) +         \