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Subject: Re: [PATCH 2/5] f2fs: clear the remaining prefree_map of the section
To:     Chao Yu <yuchao0@huawei.com>, <jaegeuk@kernel.org>,
        <chao@kernel.org>, <yunlong.song@icloud.com>
CC:     <miaoxie@huawei.com>, <bintian.wang@huawei.com>,
        <shengyong1@huawei.com>, <heyunlei@huawei.com>,
        <linux-f2fs-devel@lists.sourceforge.net>,
        <linux-kernel@vger.kernel.org>
References: <1531408170-45758-1-git-send-email-yunlong.song@huawei.com>
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 <12f3c3d2-6586-ca73-8770-8f97b436d643@huawei.com>
From:   Yunlong Song <yunlong.song@huawei.com>
Message-ID: <52acc420-2a08-c5e9-f63e-74d63fb11126@huawei.com>
Date:   Fri, 13 Jul 2018 11:51:59 +0800
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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