linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: stop iterating f2fs_map

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Message-ID: <c70b330a-b5f5-72d9-1190-fe1a6872919d@kernel.org>
Date:   Sat, 7 Oct 2023 15:40:36 +0800
From:   Chao Yu <chao@...nel.org>
To:     Jaegeuk Kim <jaegeuk@...nel.org>, linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH] f2fs: stop iterating f2fs_map_block if hole
 exists

On 2023/10/4 6:52, Jaegeuk Kim wrote:
> Let's avoid unnecessary f2fs_map_block calls to load extents.
> 
>   # f2fs_io fadvise willneed 0 4096 /data/local/tmp/test
> 
>    f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 386, start blkaddr = 0x34ac00, len = 0x1400, flags = 2,
>    f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 5506, start blkaddr = 0x34c200, len = 0x1000, flags = 2,
>    f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 9602, start blkaddr = 0x34d600, len = 0x1200, flags = 2,
>    f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 14210, start blkaddr = 0x34ec00, len = 0x400, flags = 2,
>    f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 15235, start blkaddr = 0x34f401, len = 0xbff, flags = 2,
>    f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 18306, start blkaddr = 0x350200, len = 0x1200, flags = 2
>    f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 22915, start blkaddr = 0x351601, len = 0xa7d, flags = 2
>    f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 25600, start blkaddr = 0x351601, len = 0x0, flags = 0
>    f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 25601, start blkaddr = 0x351601, len = 0x0, flags = 0
>    f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 25602, start blkaddr = 0x351601, len = 0x0, flags = 0
>    ...
>    f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 1037188, start blkaddr = 0x351601, len = 0x0, flags = 0
>    f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 1038206, start blkaddr = 0x351601, len = 0x0, flags = 0
>    f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 1039224, start blkaddr = 0x351601, len = 0x0, flags = 0
>    f2fs_map_blocks: dev = (254,51), ino = 85845, file offset = 2075548, start blkaddr = 0x351601, len = 0x0, flags = 0

Jaegeuk,

Not sure, but it looks it's due to f2fs_precache_extents() will traverse file
w/ range [0, max_file_blocks), since data which exceeds EOF will always be zero,
so it's not necessary to precache its mapping info, so we'd better adjust upper
boundary to i_size rather than max_file_blocks().

> 
> Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
> ---
>   fs/f2fs/file.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> index 161826c6e200..2403fd1de5a0 100644
> --- a/fs/f2fs/file.c
> +++ b/fs/f2fs/file.c
> @@ -3270,7 +3270,7 @@ int f2fs_precache_extents(struct inode *inode)
>   		f2fs_down_write(&fi->i_gc_rwsem[WRITE]);
>   		err = f2fs_map_blocks(inode, &map, F2FS_GET_BLOCK_PRECACHE);
>   		f2fs_up_write(&fi->i_gc_rwsem[WRITE]);
> -		if (err)
> +		if (err || !map.m_len)
Well, if there is a hole in the head of file, it may break here rather
than precaching following valid map info.

What about passing parameter offset|len from f2fs_file_fadvise() to
f2fs_precache_extents(), and then precaching mapping info on demand.

Thanks,

>   			return err;
>   
>   		map.m_lblk = m_next_extent;