Re: [PATCH v2 08/10] btrfs: relocation: Remove the open-coded goto loop for breadth-first search

[Nikolay Borisov <nborisov@suse.com>]

On 5.03.20 г. 2:40 ч., Qu Wenruo wrote:
> 
> 
> On 2020/3/4 下午10:24, Nikolay Borisov wrote:
>>
> [...]
>>> +	int err = 0;
>>> +
>>> +	iter = btrfs_backref_iter_alloc(rc->extent_root->fs_info, GFP_NOFS);
>>> +	if (!iter)
>>> +		return ERR_PTR(-ENOMEM);
>>
>> This iterator can be made private to handle_one_tree_block as I don't see it being used outside of that function.
> 
> It's kinda a performance optimization.
> 
> Instead of allocating memory for each loop, we allocate the memory just
> once, and reuse it until the whole backref map for the bytenr is built.
>>
>>> +	path = btrfs_alloc_path();
>>> +	if (!path) {
>>> +		err = -ENOMEM;
>>> +		goto out;
>>> +	}
>>
>> Same thing with this path. Overall this will reduce the argument to handle_one_tree_block by 2.
> 
> Same performance optimization here.

Ok, fair point.
> 
>>

<snip>

>>
>> or simply if (!edge)
>> break;
>>
>> Also this loop can be rewritten as a do {} while() and it will look:
> 
> Yep, but I'm not sure if such do {} while() loop is preferred.
> IIRC there are some docs saying to avoid such loop?

I'm not aware of any such docs, can you point me to them?

> 
> If there is no such restriction, I would be pretty happy to go that way.
> 
> Thanks,
> Qu
> 

<snip>