linux-kernel - RE:(2) [PATCH] f2fs: avoid unused block when dio write in LFS mode

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Message-ID: <20240816001700epcms2p37f406e9cd3a1d6438a866e7a7a3d5c31@epcms2p3>
Date: Fri, 16 Aug 2024 09:17:00 +0900
From: Daejun Park <daejun7.park@...sung.com>
To: Chao Yu <chao@...nel.org>, "jaegeuk@...nel.org" <jaegeuk@...nel.org>,
	"linux-f2fs-devel@...ts.sourceforge.net"
	<linux-f2fs-devel@...ts.sourceforge.net>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
CC: Seokhwan Kim <sukka.kim@...sung.com>, Dongjin Kim
	<dongjin_.kim@...sung.com>, Yonggil Song <yonggil.song@...sung.com>, Jaeyoon
	Choi <j_yoon.choi@...sung.com>, Jeuk Kim <jeuk20.kim@...sung.com>, Nayeon
	Kim <nayeoni.kim@...sung.com>, Siwoo Jung <siu.jung@...sung.com>, Daejun
	Park <daejun7.park@...sung.com>
Subject: RE:(2) [PATCH] f2fs: avoid unused block when dio write in LFS mode

Hi Chao Yu,
> 
>--------- Original Message ---------
>Sender : Chao Yu <chao@...nel.org>
>Date : 2024-08-14 18:50 (GMT+9)
>Title : Re: [PATCH] f2fs: avoid unused block when dio write in LFS mode
> 
>On 2024/8/1 15:47, Daejun Park wrote:
>> This patch addresses the problem that when using LFS mode, unused blocks
>> may occur in f2fs_map_blocks() during block allocation for dio writes.
>>
>> If a new section is allocated during block allocation, it will not be
>> included in the map struct by map_is_mergeable() if the LBA of the
>
>I didn't get it, why below condition in map_is_mergeable() can not catch this
>case? Can you please explain more?
>
>       if (map->m_pblk != NEW_ADDR && blkaddr == (map->m_pblk + ofs))
>               return true;
>

Thank you for your review.
map_is_mergeable() returns true when the last block in the section is merged.
The problem is the next block allocation, which happens consecutively.
Since it will be allocated a new section, its block will be the first 
block in the section.
If the newly allocated section is not contiguous with the previous section,
map_is_mergeable() will return false.
So the block is allocated but unused.
However, it is not freed, so the block is unusable.
If my explanation was not clear enough, please feel free to ask questions.

Thanks
Daejun

>Thanks,
>
>> allocated block is not contiguous. However, the block already allocated
>> in this process will remain unused due to the LFS mode.
>>
>> This patch avoids the possibility of unused blocks by escaping
>> f2fs_map_blocks() when allocating the last block in a section.
>>
>> Signed-off-by: Daejun Park <daejun7.park@...sung.com>
>> ---
>>  fs/f2fs/data.c 13 +++++++++++++
>>  1 file changed, 13 insertions(+)
>>
>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>> index b6dcb3bcaef7..b27a3f448f32 100644
>> --- a/fs/f2fs/data.c
>> +++ b/fs/f2fs/data.c
>> @@ -1711,6 +1711,19 @@ int f2fs_map_blocks(struct inode *inode, struct f2fs_map_blocks *map, int flag)
>>                   dn.ofs_in_node = end_offset;
>>           }
>> 
>> +        if (flag == F2FS_GET_BLOCK_DIO && f2fs_lfs_mode(sbi)) {
>> +                int segno = GET_SEGNO(sbi, blkaddr);
>> +                bool last_seg, last_blk;
>> +
>> +                last_seg = !((segno + 1) % SEGS_PER_SEC(sbi));
>> +                last_blk = (f2fs_usable_blks_in_seg(sbi, segno) - 1) ==
>> +                                GET_BLKOFF_FROM_SEG0(sbi, blkaddr);
>> +
>> +                /* LBA of the next block to be allocated may not be contiguous. */
>> +                if (last_seg && last_blk)
>> +                        goto sync_out;
>> +        }
>> +
>>           if (pgofs >= end)
>>                   goto sync_out;
>>           else if (dn.ofs_in_node < end_offset)