btrfs: Document parameters of btrfs_reserve_extent
This function is the entry to the extent allocator and as such has
quite a number of parameters. Some of those have subtle effects on the
allocation algorithm. Document the parameters.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 1469117..0e46142 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -7968,6 +7968,51 @@ static void dump_space_info(struct btrfs_fs_info *fs_info,
up_read(&info->groups_sem);
}
+/*
+ * btrfs_reserve_extent - entry point to the extent allocator. Tries to find a
+ * hole that is at least as big as @num_bytes.
+ *
+ * @root - The root that will contain this extent
+ *
+ * @ram_bytes - The amount of space in ram that @num_bytes take. This
+ * is used for accounting purposes. This value differs
+ * from @num_bytes only in the case of compressed extents.
+ *
+ * @num_bytes - Number of bytes to allocate on-disk.
+ *
+ * @min_alloc_size - Indicates the minimum amount of space that the
+ * allocator should try to satisfy. In some cases
+ * @num_bytes may be larger than what is required and if
+ * the filesystem is fragmented then allocation fails.
+ * However, the presence of @min_alloc_size gives a
+ * chance to try and satisfy the smaller allocation.
+ *
+ * @empty_size - A hint that you plan on doing more COW. This is the
+ * size in bytes the allocator should try to find free
+ * next to the block it returns. This is just a hint and
+ * may be ignored by the allocator.
+ *
+ * @hint_byte - Hint to the allocator to start searching above the byte
+ * address passed. It might be ignored.
+ *
+ * @ins - This key is modified to record the found hole. It will
+ * have the following values:
+ * ins->objectid == start position
+ * ins->flags = BTRFS_EXTENT_ITEM_KEY
+ * ins->offset == the size of the hole.
+ *
+ * @is_data - Boolean flag indicating whether an extent is
+ * allocated for data (true) or metadata (false)
+ *
+ * @delalloc - Boolean flag indicating whether this allocation is for
+ * delalloc or not. If 'true' data_rwsem of block groups
+ * is going to be acquired.
+ *
+ *
+ * Returns 0 when an allocation succeeded or < 0 when an error occurred. In
+ * case -ENOSPC is returned then @ins->offset will contain the size of the
+ * largest available hole the allocator managed to find.
+ */
int btrfs_reserve_extent(struct btrfs_root *root, u64 ram_bytes,
u64 num_bytes, u64 min_alloc_size,
u64 empty_size, u64 hint_byte,