Re: [PATCH 4/7] btrfs: use the global rsv size in the preemptive thresh calculation

[Josef Bacik <josef@toxicpanda.com>]

On 4/29/21 10:04 AM, Johannes Thumshirn wrote:
> On 28/04/2021 19:40, Josef Bacik wrote:
>> -	thresh += (space_info->total_bytes - space_info->bytes_used -
>> -		   space_info->bytes_reserved - space_info->bytes_readonly);
>> +	used = space_info->bytes_used + space_info->bytes_reserved +
>> +		space_info->bytes_readonly + global_rsv_size;
>> +	if (used < space_info->total_bytes)
>> +		thresh += space_info->total_bytes - used;
>>   	thresh >>= space_info->clamp;
> 
> 
> I don't quite understand why you introduced the 'if' here. Now we're only
> adding the new free space to the threshold if we're using less than our
> total free space, which kinda makes sense that we're not going over our
> total free space.
> 

Because it's possible that the global_rsv_size + used is > total_bytes, because 
sometimes the global reserve can end up being calculated as larger than the 
available size (think stupid small fs'es where we only have the original 8mib 
chunk of metadata).  It doesn't usually happen, but that sort of thinking gets 
me into trouble, so this is safer.  Thanks,

Josef