Re: [PATCH] btrfs: transaction: Commit transaction more frequently for BPF

[David Sterba <dsterba@suse.cz>]

On Tue, Aug 06, 2019 at 04:22:01PM +0800, Qu Wenruo wrote:
> Btrfs has btrfs_end_transaction_throttle() which could try to commit
> transaction when needed.
> 
> However under most cases btrfs_end_transaction_throttle() won't really
> commit transaction, due to the hard timing requirement.
> 
> Now introduce a new error injection point, btrfs_need_trans_pressure(),
> to allow btrfs_should_end_transaction() to return 1 and
> btrfs_end_transaction_throttle() to fallback to
> btrfs_commit_transaction().
> 
> With such more aggressive transaction commit, we can dig deeper into
> cases like snapshot drop.
> Now each reference drop of btrfs_drop_snapshot() will lead to a
> transaction commit, allowing dm-logwrites to catch more details, other
> than one big transaction dropping everything.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/transaction.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> index 248d535bb14d..2e758957126e 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -10,6 +10,7 @@
>  #include <linux/pagemap.h>
>  #include <linux/blkdev.h>
>  #include <linux/uuid.h>
> +#include <linux/error-injection.h>
>  #include "ctree.h"
>  #include "disk-io.h"
>  #include "transaction.h"
> @@ -781,10 +782,18 @@ void btrfs_throttle(struct btrfs_fs_info *fs_info)
>  	wait_current_trans(fs_info);
>  }
>  

Please put a comment here what's the purpose of the function otherwise
the people who like to delete code will find it and you know what will
happen next.

> +static noinline bool btrfs_need_trans_pressure(struct btrfs_trans_handle *trans)
> +{
> +	return false;
> +}
> +ALLOW_ERROR_INJECTION(btrfs_need_trans_pressure, TRUE);
> +
>  static int should_end_transaction(struct btrfs_trans_handle *trans)
>  {
>  	struct btrfs_fs_info *fs_info = trans->fs_info;
>  
> +	if (btrfs_need_trans_pressure(trans))
> +		return 1;
>  	if (btrfs_check_space_for_delayed_refs(fs_info))
>  		return 1;
>  
> @@ -845,6 +854,8 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
>  
>  	btrfs_trans_release_chunk_metadata(trans);
>  
> +	if (throttle && btrfs_need_trans_pressure(trans))
> +		return btrfs_commit_transaction(trans);
>  	if (lock && READ_ONCE(cur_trans->state) == TRANS_STATE_BLOCKED) {
>  		if (throttle)
>  			return btrfs_commit_transaction(trans);
> -- 
> 2.22.0