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-rw-r--r--fs/ext4/extents.c30
1 files changed, 26 insertions, 4 deletions
diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 0554c48..d53e20f 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -117,11 +117,33 @@ static ext4_fsblk_t ext4_ext_find_goal(struct inode *inode,
struct ext4_extent *ex;
depth = path->p_depth;
- /* try to predict block placement */
+ /*
+ * Try to predict block placement assuming that we are
+ * filling in a file which will eventually be
+ * non-sparse --- i.e., in the case of libbfd writing
+ * an ELF object sections out-of-order but in a way
+ * the eventually results in a contiguous object or
+ * executable file, or some database extending a table
+ * space file. However, this is actually somewhat
+ * non-ideal if we are writing a sparse file such as
+ * qemu or KVM writing a raw image file that is going
+ * to stay fairly sparse, since it will end up
+ * fragmenting the file system's free space. Maybe we
+ * should have some hueristics or some way to allow
+ * userspace to pass a hint to file system,
+ * especiially if the latter case turns out to be
+ * common.
+ */
ex = path[depth].p_ext;
- if (ex)
- return (ext4_ext_pblock(ex) +
- (block - le32_to_cpu(ex->ee_block)));
+ if (ex) {
+ ext4_fsblk_t ext_pblk = ext4_ext_pblock(ex);
+ ext4_lblk_t ext_block = le32_to_cpu(ex->ee_block);
+
+ if (block > ext_block)
+ return ext_pblk + (block - ext_block);
+ else
+ return ext_pblk - (ext_block - block);
+ }
/* it looks like index is empty;
* try to find starting block from index itself */
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