Commit ecd84d54 authored by Filipe Manana's avatar Filipe Manana Committed by David Sterba
Browse files

btrfs: update comments for chunk allocation -ENOSPC cases 



Update the comments at btrfs_chunk_alloc() and do_chunk_alloc() that
describe which cases can lead to a failure to allocate metadata and system
space despite having previously reserved space. This adds one more reason
that I previously forgot to mention.

Reviewed-by: default avatarJosef Bacik <josef@toxicpanda.com>
Signed-off-by: default avatarFilipe Manana <fdmanana@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent 2bb2e00e

fs/btrfs/block-group.c

+18 −3
Original line number Diff line number Diff line
@@ -3429,7 +3429,7 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans, u64 flags) 
	/*

	 * Normally we are not expected to fail with -ENOSPC here, since we have

	 * previously reserved space in the system space_info and allocated one

	 * new system chunk if necessary. However there are two exceptions:

	 * new system chunk if necessary. However there are three exceptions:

	 *

	 * 1) We may have enough free space in the system space_info but all the

	 *    existing system block groups have a profile which can not be used
@@ -3455,7 +3455,14 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans, u64 flags) 
	 *    with enough free space got turned into RO mode by a running scrub,

	 *    and in this case we have to allocate a new one and retry. We only

	 *    need do this allocate and retry once, since we have a transaction

	 *    handle and scrub uses the commit root to search for block groups.

	 *    handle and scrub uses the commit root to search for block groups;

	 *

	 * 3) We had one system block group with enough free space when we called

	 *    check_system_chunk(), but after that, right before we tried to

	 *    allocate the last extent buffer we needed, a discard operation came

	 *    in and it temporarily removed the last free space entry from the

	 *    block group (discard removes a free space entry, discards it, and

	 *    then adds back the entry to the block group cache).

	 */

	if (ret == -ENOSPC) {

		const u64 sys_flags = btrfs_system_alloc_profile(trans->fs_info);
@@ -3539,7 +3546,15 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans, u64 flags) 
 *    properly, either intentionally or as a bug. One example where this is

 *    done intentionally is fsync, as it does not reserve any transaction units

 *    and ends up allocating a variable number of metadata extents for log

 *    tree extent buffers.

 *    tree extent buffers;

 *

 * 4) The task has reserved enough transaction units / metadata space, but right

 *    before it tries to allocate the last extent buffer it needs, a discard

 *    operation comes in and, temporarily, removes the last free space entry from

 *    the only metadata block group that had free space (discard starts by

 *    removing a free space entry from a block group, then does the discard

 *    operation and, once it's done, it adds back the free space entry to the

 *    block group).

 *

 * We also need this 2 phases setup when adding a device to a filesystem with

 * a seed device - we must create new metadata and system chunks without adding