Re: [RFC PATCH v4 12/17] fs: Optimization for concurrent file time updates.

[Jan Kara <jack@suse.cz>]

On Fri 20-05-22 11:36:41, Stefan Roesch wrote:
> This introduces the S_PENDING_TIME flag. If an async buffered write
> needs to update the time, it cannot be processed in the fast path of
> io-uring. When a time update is pending this flag is set for async
> buffered writes. Other concurrent async buffered writes for the same
> file do not need to wait while this time update is pending.
> 
> This reduces the number of async buffered writes that need to get punted
> to the io-workers in io-uring.
> 
> Signed-off-by: Stefan Roesch <shr@fb.com>

...

> @@ -2184,10 +2184,17 @@ int file_modified_async(struct file *file, int flags)
>  	ret = file_needs_update_time(inode, file, &now);
>  	if (ret <= 0)
>  		return ret;
> -	if (flags & IOCB_NOWAIT)
> +	if (flags & IOCB_NOWAIT) {
> +		if (IS_PENDING_TIME(inode))
> +			return 0;
> +
> +		inode->i_flags |= S_PENDING_TIME;
>  		return -EAGAIN;
> +	}
>  
> -	return __file_update_time(inode, file, &now, ret);
> +	ret = __file_update_time(inode, file, &now, ret);
> +	inode->i_flags &= ~S_PENDING_TIME;
> +	return ret;
>  }
>  EXPORT_SYMBOL(file_modified_async);

You still didn't address my concern that i_flags is modified without the
protection of i_rwsem here. That can lead to corruption of i_flags value
and rather nasty (and hard to debug) consequences. You can use
inode_set_flags() here to make things kinda safe. The whole inode->i_flags
handling is a mess but not yours to resolve ;)

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR