Re: [PATCH] f2fs: discard small invalid blocks in current active segments

[Chao Yu]

On 2017/8/28 21:21, Yunlong Song wrote:
> How? Can the invalid blocks of file A be discarded, if file B is alive 
> all the time and fggc_threshold is 507 ?

As I traced, with small discard, we scan discard candidates from all dirty
segments which also include current segment, so seems we don't need to wait log
header moving on.

Thanks,

> 
> On 2017/8/28 17:59, Chao Yu wrote:
>> On 2017/8/26 20:49, Yunlong Song wrote:
>>> 1. write file A with 5 blocks to current empty active segment
>>> 2. remove file A
>>> 3. write checkpoint
>>> 4. write file B with 507 blocks to the same active segment
>>>
>>> If file B is alive all the time, the blocks used by file A will never be
>>> discarded. So current active segment should also be treated as a candidate
>>> for small discards.
>> I don't think that would be a big issue, since there will not be any freezing
>> log headers, once log header moves, later invalid blocks could be discarded.
>>
>> Thanks,
>>
>>> Signed-off-by: Yunlong Song <yunlong.song@xxxxxxxxxx>
>>> ---
>>>   fs/f2fs/segment.c | 4 +++-
>>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>>> index 8375257..a2e7c8f 100644
>>> --- a/fs/f2fs/segment.c
>>> +++ b/fs/f2fs/segment.c
>>> @@ -1339,7 +1339,9 @@ static bool add_discard_addrs(struct f2fs_sb_info *sbi, struct cp_control *cpc,
>>>   		return false;
>>>   
>>>   	if (!force) {
>>> -		if (!test_opt(sbi, DISCARD) || !se->valid_blocks ||
>>> +		if (!test_opt(sbi, DISCARD) ||
>>> +			(!se->valid_blocks &&
>>> +			!IS_CURSEG(sbi, cpc->trim_start)) ||
>>>   			SM_I(sbi)->dcc_info->nr_discards >=
>>>   				SM_I(sbi)->dcc_info->max_discards)
>>>   			return false;
>>>
>>
>> .
>>
>