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

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Date:   Sat, 4 Sep 2021 14:45:09 +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 2021/9/4 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.

Thanks,

>