linux-kernel - Re: [f2fs-dev] [PATCH v3 1/2] f2fs: fix zero-sized extent for precache extents

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Message-ID: <aMra2_bW9cPCRcTs@google.com>
Date: Wed, 17 Sep 2025 15:59:23 +0000
From: Jaegeuk Kim <jaegeuk@...nel.org>
To: wangzijie <wangzijie1@...or.com>
Cc: chao@...nel.org, linux-f2fs-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, bintian.wang@...or.com,
	feng.han@...or.com
Subject: Re: [f2fs-dev] [PATCH v3 1/2] f2fs: fix zero-sized extent for
 precache extents

On 09/17, wangzijie wrote:
> Script to reproduce:
> f2fs_io write 1 0 1881 rand dsync testfile
> f2fs_io fallocate 0 7708672 4096 testfile
> f2fs_io write 1 1881 1 rand buffered testfile
> fsync testfile
> umount
> mount
> f2fs_io precache_extents testfile
> 
> When the data layout is something like this:
> dnode1:                     dnode2:
> [0]      A                  [0]    NEW_ADDR
> [1]      A+1                [1]    0x0
> ...
> [1016]   A+1016
> [1017]   B (B!=A+1017)      [1017] 0x0
> 
> During precache_extents, we map the last block(valid blkaddr) in dnode1:
> map->m_flags |= F2FS_MAP_MAPPED;
> map->m_pblk = blkaddr(valid blkaddr);
> map->m_len = 1;
> then we goto next_dnode, meet the first block in dnode2(hole), goto sync_out:
> map->m_flags & F2FS_MAP_MAPPED == true, and we make zero-sized extent:
> 
> map->m_len = 1
> ofs = start_pgofs - map->m_lblk = 1882 - 1881 = 1
> ei.fofs = start_pgofs = 1882
> ei.len = map->m_len - ofs = 1 - 1 = 0
> 
> 
> Rebased on patch[1], this patch can cover these cases to avoid zero-sized extent:
> A,B,C is valid blkaddr
> case1:
> dnode1:                     dnode2:
> [0]      A                  [0]    NEW_ADDR
> [1]      A+1                [1]    0x0
> ...                         ....
> [1016]   A+1016
> [1017]   B (B!=A+1017)      [1017] 0x0
> 
> case2:
> dnode1:                     dnode2:
> [0]      A                  [0]    C (C!=B+1)
> [1]      A+1                [1]    C+1
> ...                         ....
> [1016]   A+1016
> [1017]   B (B!=A+1017)      [1017] 0x0
> 
> case3:
> dnode1:                     dnode2:
> [0]      A                  [0]    C (C!=B+2)
> [1]      A+1                [1]    C+1
> ...                         ....
> [1015]   A+1015
> [1016]   B (B!=A+1016)
> [1017]   B+1                [1017] 0x0
> 
> [1] https://lore.kernel.org/linux-f2fs-devel/20250912081250.44383-1-chao@kernel.org/
> 
> Fixes: c4020b2da4c9 ("f2fs: support F2FS_IOC_PRECACHE_EXTENTS")
> Signed-off-by: wangzijie <wangzijie1@...or.com>
> ---
> Rebased on:
> https://lore.kernel.org/linux-f2fs-devel/20250912081250.44383-1-chao@kernel.org/
> v2:
> https://lore.kernel.org/linux-f2fs-devel/20250915035246.98055-1-wangzijie1@honor.com/
> ---
>  fs/f2fs/data.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index 838eae39d..7a5170b32 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -1778,9 +1778,10 @@ int f2fs_map_blocks(struct inode *inode, struct f2fs_map_blocks *map, int flag)
>  		if (map->m_flags & F2FS_MAP_MAPPED) {
>  			unsigned int ofs = start_pgofs - map->m_lblk;
>  
> -			f2fs_update_read_extent_cache_range(&dn,
> -				start_pgofs, map->m_pblk + ofs,
> -				map->m_len - ofs);
> +			if (map->m_len - ofs > 0)

Applied with
			if (map->m_len > ofs)

> +				f2fs_update_read_extent_cache_range(&dn,
> +					start_pgofs, map->m_pblk + ofs,
> +					map->m_len - ofs);
>  		}
>  		if (map->m_next_extent)
>  			*map->m_next_extent = is_hole ? pgofs + 1 : pgofs;
> -- 
> 2.25.1