Re: [PATCH 06/35] btrfs: check if free bgs for commit

Omar Sandoval
2018-09-01T03:27:50Z

On Thu, Aug 30, 2018 at 01:41:56PM -0400, Josef Bacik wrote:
> may_commit_transaction will skip committing the transaction if we don't
> have enough pinned space or if we're trying to find space for a SYSTEM
> chunk. However if we have pending free block groups in this transaction
> we still want to commit as we may be able to allocate a chunk to make
> our reservation. So instead of just returning ENOSPC, check if we have
> free block groups pending, and if so commit the transaction to allow us
> to use that free space.

This makes sense.

> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
> fs/btrfs/extent-tree.c | 18 ++++++++++++------
> 1 file changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 6e7f350754d2..80615a579b18 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -4804,6 +4804,7 @@ static int may_commit_transaction(struct btrfs_fs_info *fs_info,
> struct btrfs_trans_handle *trans;
> u64 bytes;
> u64 reclaim_bytes = 0;
> + bool do_commit = true;

I find this naming a little mind bending when I read

do_commit = false;
goto commit;

Since the end result is that we always join the transaction if we make
it past the (!bytes) check anyways, can we do the pending bgs check
first? I find the following easier to follow, fwiw.

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index de6f75f5547b..dd7aeb5fb6bf 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -4779,18 +4779,25 @@ static int may_commit_transaction(struct btrfs_fs_info *fs_info,
if (!bytes)
return 0;

- /* See if there is enough pinned space to make this reservation */
- if (__percpu_counter_compare(&space_info->total_bytes_pinned,
- bytes,
- BTRFS_TOTAL_BYTES_PINNED_BATCH) >= 0)
- goto commit;
+ trans = btrfs_join_transaction(fs_info->extent_root);
+ if (IS_ERR(trans))
+ return -ENOSPC;
+
+ /*
+ * See if we have a pending bg or there is enough pinned space to make
+ * this reservation.
+ */
+ if (test_bit(BTRFS_TRANS_HAVE_FREE_BGS, &trans->transaction->flags) ||
+ __percpu_counter_compare(&space_info->total_bytes_pinned, bytes,
+ BTRFS_TOTAL_BYTES_PINNED_BATCH) >= 0)
+ return btrfs_commit_transaction(trans);

/*
* See if there is some space in the delayed insertion reservation for
* this reservation.
*/
if (space_info != delayed_rsv->space_info)
- return -ENOSPC;
+ goto enospc;

spin_lock(&delayed_rsv->lock);
if (delayed_rsv->size > bytes)
@@ -4801,16 +4808,14 @@ static int may_commit_transaction(struct btrfs_fs_info *fs_info,

if (__percpu_counter_compare(&space_info->total_bytes_pinned,
bytes,
- BTRFS_TOTAL_BYTES_PINNED_BATCH) < 0) {
- return -ENOSPC;
- }
-
-commit:
- trans = btrfs_join_transaction(fs_info->extent_root);
- if (IS_ERR(trans))
- return -ENOSPC;
+ BTRFS_TOTAL_BYTES_PINNED_BATCH) < 0)
+ goto enospc;

return btrfs_commit_transaction(trans);
+
+enospc:
+ btrfs_end_transaction(trans);
+ return -ENOSPC;
}

/*