Re: [PATCH 1/2] nilfs2: add nilfs_sufile_trim_fs to trim clean segs

[Andreas Rohner <andreas.rohner-hi6Y0CQ0nG0@public.gmane.org>]

On 2014-02-17 11:06, Ryusuke Konishi wrote:
> On Mon, 17 Feb 2014 10:17:57 +0100, Andreas Rohner wrote:
>> On 2014-02-17 04:00, Ryusuke Konishi wrote:
>>>> +	if (end > nilfs->ns_nsegments)
>>>> +		end = nilfs->ns_nsegments;
>>>
>>> Yes, this adjustment is what we should do here, but 'end' segnum was
>>> rounded down to segment alighment before.  So, it should be:
>>>
>>>   if (end >= nilfs->ns_nsegments)
>>> 	end = nilfs->ns_nsegments - 1;
>>>
>>>> +	if (end == segnum)
>>>> +		goto out;
>>>
>>> and
>>>
>>>   if (end < segnum)
>>> 	goto out;
>>>
>>>> +
>>>> +	down_read(&NILFS_MDT(sufile)->mi_sem);
>>>> +
>>>> +	while (segnum < end) {
>>>
>>> and
>>>
>>>   while (segnum <= end) {
>>>
>>>> +		n = min_t(unsigned long,
>>>> +			  segusages_per_block -
>>>> +				  nilfs_sufile_get_offset(sufile, segnum),
>>>> +			  end - segnum);
>>>
>>> Then, we can reuse nilfs_sufile_segment_usages_in_block() to calculate
>>> 'n'.
>>
>> Actually I don't think that is correct. What if range->start = 0 and
>> range->end = 8MB. Then segnum = 0 and end = 1. Your code would discard
>> segment 0 and segment 1, whereas my version would discard only segment
>> 0, which seems to be more reasonable.
> 
> The problem seems that 'end' is not calculated properly.
> I think it should be
> 
> end = nilfs_get_segnum_of_block(
> 		nilfs,
> 		(range->start + range->len + <segment-size-in-bytes> - 1)
> 		  >> nilfs->ns_blocksize_bits) - 1;
> 
>  or can be simplified to
> 
> end = nilfs_get_segnum_of_block(
> 		nilfs,
> 		(range->start + range->len - 1) >> nilfs->ns_blocksize_bits);
> 
>  if range->len > 0 is guaranteed.
> 
> 
> The calculation of segnum extends the range to be discarded since it
> is rounded down to segment alignment, but that of the current
> implementation for 'end' truncates the range.  Both should be
> extended.

Then shouldn't both be truncated? The user expects that less bytes than
range->len are discarded but not more. Maybe I am wrong and it is
defined somewhere...

Regards,
Andreas Rohner
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