Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit 6f47c706 authored by Nikolay Borisov's avatar Nikolay Borisov Committed by David Sterba
Browse files

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: default avatarNikolay Borisov <nborisov@suse.com>
Reviewed-by: default avatarAnand Jain <anand.jain@oracle.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent d87ff758

fs/btrfs/extent-tree.c

+45 −0
Original line number Diff line number Diff line
@@ -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,