btrfs: allow defrag to convert inline extents to regular extents

Btrfs defaults to max_inline=2K to make small writes inlined into
metadata.

The default value is always a win, as even DUP/RAID1/RAID10 doubles the
metadata usage, it should still cause less physical space used compared
to a 4K regular extents.

But since the introduction of RAID1C3 and RAID1C4 it's no longer the case,
users may find inlined extents causing too much space wasted, and want
to convert those inlined extents back to regular extents.

Unfortunately defrag will unconditionally skip all inline extents, no
matter if the user is trying to converting them back to regular extents.

So this patch will add a small exception for defrag_collect_targets() to
allow defragging inline extents, if and only if the inlined extents are
larger than max_inline, allowing users to convert them to regular ones.

This also allows us to defrag extents like the following:

	item 6 key (257 EXTENT_DATA 0) itemoff 15794 itemsize 69
		generation 7 type 0 (inline)
		inline extent data size 48 ram_bytes 4096 compression 1 (zlib)
	item 7 key (257 EXTENT_DATA 4096) itemoff 15741 itemsize 53
		generation 7 type 1 (regular)
		extent data disk byte 13631488 nr 4096
		extent data offset 0 nr 16384 ram 16384
		extent compression 1 (zlib)

Previously we're unable to do any defrag, since the first extent is
inlined, and the second one has no extent to merge.

Now we can defrag it to just one single extent, saving 48 bytes metadata
space.

	item 6 key (257 EXTENT_DATA 0) itemoff 15810 itemsize 53
		generation 8 type 1 (regular)
		extent data disk byte 13635584 nr 4096
		extent data offset 0 nr 20480 ram 20480
		extent compression 1 (zlib)
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -1420,8 +1420,19 @@ static int defrag_collect_targets(struct btrfs_inode *inode,
 		if (!em)
 			break;
 
-		/* Skip hole/inline/preallocated extents */
-		if (em->block_start >= EXTENT_MAP_LAST_BYTE ||
+		/*
+		 * If the file extent is an inlined one, we may still want to
+		 * defrag it (fallthrough) if it will cause a regular extent.
+		 * This is for users who want to convert inline extents to
+		 * regular ones through max_inline= mount option.
+		 */
+		if (em->block_start == EXTENT_MAP_INLINE &&
+		    em->len <= inode->root->fs_info->max_inline)
+			goto next;
+
+		/* Skip hole/delalloc/preallocated extents */
+		if (em->block_start == EXTENT_MAP_HOLE ||
+		    em->block_start == EXTENT_MAP_DELALLOC ||
 		    test_bit(EXTENT_FLAG_PREALLOC, &em->flags))
 			goto next;
 
@@ -1480,6 +1491,15 @@ static int defrag_collect_targets(struct btrfs_inode *inode,
 		if (em->len >= get_extent_max_capacity(em))
 			goto next;
 
+		/*
+		 * Normally there are no more extents after an inline one, thus
+		 * @next_mergeable will normally be false and not defragged.
+		 * So if an inline extent passed all above checks, just add it
+		 * for defrag, and be converted to regular extents.
+		 */
+		if (em->block_start == EXTENT_MAP_INLINE)
+			goto add;
+
 		next_mergeable = defrag_check_next_extent(&inode->vfs_inode, em,
 						extent_thresh, newer_than, locked);
 		if (!next_mergeable) {