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SubjectRe: [PATCH 2/5] f2fs: clear the remaining prefree_map of the section
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Because in f2fs_clear_prefree_segments, the codes:
...
while (1) {
int i;
start = find_next_bit(prefree_map, MAIN_SEGS(sbi), end + 1);
if (start >= MAIN_SEGS(sbi))
break;
end = find_next_zero_bit(prefree_map, MAIN_SEGS(sbi),
start + 1);

for (i = start; i < end; i++)
clear_bit(i, prefree_map);
...
next:
secno = GET_SEC_FROM_SEG(sbi, start);
start_segno = GET_SEG_FROM_SEC(sbi, secno);
if (!IS_CURSEC(sbi, secno) &&
!get_valid_blocks(sbi, start, true))
f2fs_issue_discard(sbi, START_BLOCK(sbi, start_segno),
sbi->segs_per_sec << sbi->log_blocks_per_seg);

start = start_segno + sbi->segs_per_sec;
if (start < end)
goto next;
else
end = start - 1;
...
In round 2, for prefree_map: 1 1 0 1 1, start = 0, end = 2, then

start = start_segno + sbi->segs_per_sec makes start = 5

if (start < end) --> start = 5, end = 2

so end = start -1 --> end = 4, then return to while again, this time skips
prefree bit 3 and 4.

On 2018/7/13 11:42, Chao Yu wrote:
> On 2018/7/13 11:28, Yunlong Song wrote:
>> round 1: section bitmap : 1 1 1 1 1, all valid, prefree_map: 0 0 0 0 0
>> then rm data block NO.2, block NO.2 becomes invalid, prefree_map: 0 0 1 0 0
>> write_checkpoint: section bitmap: 1 1 0 1 1, prefree_map: 0 0 0 0 0,
>> prefree of NO.2 is cleared, and no discard issued
>>
>> round2: rm data block NO.0, NO.1, NO.3, NO.4
>> all invalid, but prefree bit of NO.2 is set and cleared in round1, then
>> prefree_map: 1 1 0 1 1
>> write_checkpoint: section bitmap: 0 0 0 0 0, prefree_map: 0 0 0 1 1, no
> Why prefree_map is not 0 0 0 0 0?
>
> Thanks,
>
>> valid blocks of this section, so discard issued
>> but this time prefree bit of NO.3 and NO.4 is skipped...
>>
>> round3:
>> write_checkpoint: section bitmap: 0 0 0 0 0, prefree_map: 0 0 0 1 1 - >
>> 0 0 0 0 0, no valid blocks of this section, so discard issued
>> this time prefree bit of NO.3 and NO.4 is cleared, but the discard of
>> this section is sent again...
>>
>> On 2018/7/13 11:13, Chao Yu wrote:
>>> On 2018/7/12 23:09, Yunlong Song wrote:
>>>> For the case when sbi->segs_per_sec > 1, take section:segment = 5 for
>>>> example, if the section prefree_map is ...previous section | current
>>>> section (1 1 0 1 1) | next section..., then the start = x, end = x + 1,
>>>> after start = start_segno + sbi->segs_per_sec, start = x + 5, then it
>>>> will skip x + 3 and x + 4, but their bitmap is still set, which will
>>>> cause duplicated f2fs_issue_discard of this same section in the next
>>>> write_checkpoint, so fix it.
>>> I didn't get it, if # 2 segment is not prefree state, so it still has valid
>>> blocks there, so we won't issue discard due to below condition, right?
>>>
>>> if (!IS_CURSEC(sbi, secno) &&
>>> !get_valid_blocks(sbi, start, true))
>>>
>>> Thanks,
>>>
>>>> Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
>>>> ---
>>>> fs/f2fs/segment.c | 19 +++++++++++++++++--
>>>> 1 file changed, 17 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>>>> index 47b6595..fd38b61 100644
>>>> --- a/fs/f2fs/segment.c
>>>> +++ b/fs/f2fs/segment.c
>>>> @@ -1684,8 +1684,23 @@ void f2fs_clear_prefree_segments(struct f2fs_sb_info *sbi,
>>>> start = start_segno + sbi->segs_per_sec;
>>>> if (start < end)
>>>> goto next;
>>>> - else
>>>> - end = start - 1;
>>>> + else {
>>>> + start_segno = start;
>>>> +
>>>> + while (1) {
>>>> + start = find_next_bit(prefree_map, start_segno,
>>>> + end + 1);
>>>> + if (start >= start_segno)
>>>> + break;
>>>> + end = find_next_zero_bit(prefree_map, start_segno,
>>>> + start + 1);
>>>> + for (i = start; i < end; i++)
>>>> + clear_bit(i, prefree_map);
>>>> + dirty_i->nr_dirty[PRE] -= end - start;
>>>> + }
>>>> +
>>>> + end = start_segno - 1;
>>>> + }
>>>> }
>>>> mutex_unlock(&dirty_i->seglist_lock);
>>>>
>>>>
>>> .
>>>
>
> .
>

--
Thanks,
Yunlong Song


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