linux-kernel - Re: [f2fs-dev] [PATCH v4] f2fs: introduce fragment allocation mode mount option

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Date:   Sat, 11 Sep 2021 18:42:01 +0800
From:   Chao Yu <chao@...nel.org>
To:     Daeho Jeong <daeho43@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, kernel-team@...roid.com,
        Daeho Jeong <daehojeong@...gle.com>
Subject: Re: [f2fs-dev] [PATCH v4] f2fs: introduce fragment allocation mode
 mount option

On 2021/9/10 23:24, Daeho Jeong wrote:
> On Fri, Sep 10, 2021 at 7:34 AM Daeho Jeong <daeho43@...il.com> wrote:
>>
>> On Thu, Sep 9, 2021 at 4:50 PM Chao Yu <chao@...nel.org> wrote:
>>>
>>> On 2021/9/8 2:12, Daeho Jeong wrote:
>>>> On Fri, Sep 3, 2021 at 11:45 PM Chao Yu <chao@...nel.org> wrote:
>>>>>
>>>>> On 2021/9/4 12:40, Daeho Jeong wrote:
>>>>>>> As a per curseg field.
>>>>>>>
>>>>>>>> Maybe, we run into the same race condition issue you told before for
>>>>>>>> fragment_remained_chunk.
>>>>>>>> Could you clarify this more?
>>>>>>>
>>>>>>> e.g.
>>>>>>>
>>>>>>> F2FS_OPTION(sbi).fs_mode = FS_MODE_FRAGMENT_FIXED_BLK
>>>>>>> fragment_chunk_size = 384
>>>>>>> fragment_hole_size = 384
>>>>>>>
>>>>>>> When creating hole:
>>>>>>>
>>>>>>> - f2fs_allocate_data_block
>>>>>>>      - __refresh_next_blkoff
>>>>>>>        chunk locates in [0, 383] of current segment
>>>>>>>        seg->next_blkoff = 384
>>>>>>>        sbi->fragment_remained_chunk = 0
>>>>>>>        then we will reset sbi->fragment_remained_chunk to 384
>>>>>>>        and move seg->next_blkoff forward to 768 (384 + 384)
>>>>>>>      - __has_curseg_space() returns false
>>>>>>>      - allocate_segment() allocates new current segment
>>>>>>>
>>>>>>> So, for such case that hole may cross two segments, hole size may be truncated
>>>>>>> to left size of previous segment.
>>>>>>
>>>>>> First, sbi->fragment_remained_chunk should be seg->fragment_remained_chunk.
>>>>>
>>>>> Oh, correct.
>>>>>
>>>>>> I understand what you mean, so you mean we need to take the leftover
>>>>>> "hole" size over to the next segment?
>>>>>> In the example, the leftover hole size will be (384 - (512-384)). Do
>>>>>> you want to take this over to the next segment?
>>>>>
>>>>> Yes, the left 256 block-sized hole should be created before next chunk
>>>>> in next opened segment.
>>>>>
>>>>
>>>> Chao,
>>>>
>>>> Do you have any decent idea to pass the left hole size to the next
>>>> segment which will be allocated?
>>>
>>> Daeho,
>>>
>>> I guess we can record left hole size in seg->fragment_remained_hole.
>>>
>>
>> I understand we need a new fragment_remained_hole variable in segment structure.
>> But, I mean.. How can we pass over the left hole size from the
>> previous segment to the next segment?
>>
> 
> I mean we don't know which segment will be the next segment, do we?

Yeah, that's why I prefer to let __get_next_segno() return zero in fixed_block
fragment mode, then log header may have chance to allocate hole in contiguous
segments.

Thanks,

> 
>> Thanks,
>>
>>> Thanks,
>>>
>>>>
>>>> Thanks,
>>>>
>>>>> Thanks,
>>>>>
>>>>>>