linux-kernel - Re: [PATCH v2] f2fs: fix missing discard candidates in fstrim

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Message-ID: <77fa4633-f7db-4daa-a9e1-3fdb5bf9dd1d@kernel.org>
Date: Mon, 20 Jan 2025 19:45:54 +0800
From: Chao Yu <chao@...nel.org>
To: Chunhai Guo <guochunhai@...o.com>, jaegeuk@...nel.org
Cc: chao@...nel.org, linux-f2fs-devel@...ts.sourceforge.net,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] f2fs: fix missing discard candidates in fstrim

On 1/19/25 22:08, Chunhai Guo wrote:
> fstrim may miss candidates that need to be discarded, as shown in the
> examples below.
> 
> The root cause is that when cpc->reason is set with CP_DISCARD,
> add_discard_addrs() expects that ckpt_valid_map and cur_valid_map have
> been synced by seg_info_to_raw_sit() [1], and it tries to find the
> candidates based on ckpt_valid_map and discard_map. However,
> seg_info_to_raw_sit() does not actually run before
> f2fs_exist_trim_candidates(), resulting in the failure.
> 
> The code logic can be simplified for all cases by finding all the
> discard blocks based only on discard_map. This might result in more
> discard blocks being sent for the segment during the first checkpoint
> after mounting, which were originally expected to be sent only in
> fstrim. Regardless, these discard blocks should eventually be sent, and
> the simplified code makes sense in this context.
> 
> root# cp testfile /f2fs_mountpoint
> 
> root# f2fs_io fiemap 0 1 /f2fs_mountpoint/testfile
> Fiemap: offset = 0 len = 1
>          logical addr.    physical addr.   length           flags
> 0       0000000000000000 0000000406a00000 000000003d800000 00001000
> 
> root# rm /f2fs_mountpoint/testfile
> 
> root# fstrim -v -o 0x406a00000 -l 1024M /f2fs_mountpoint -- no candidate is found
> /f2fs_mountpoint: 0 B (0 bytes) trimmed
> 
> Relevant code process of the root cause:
> f2fs_trim_fs()
>      f2fs_write_checkpoint()
>          ...
>          if (cpc->reason & CP_DISCARD) {
>                  if (!f2fs_exist_trim_candidates(sbi, cpc)) {
>                      unblock_operations(sbi);
>                      goto out; // No candidates are found here, and it exits.
>                  }
>              ...
>          }
> 
> [1] Please refer to commit d7bc2484b8d4 ("f2fs: fix small discards not
> to issue redundantly") for the relationship between
> seg_info_to_raw_sit() and add_discard_addrs().
> 
> Fixes: 25290fa5591d ("f2fs: return fs_trim if there is no candidate")
> Signed-off-by: Chunhai Guo <guochunhai@...o.com>
> ---
> v1: https://lore.kernel.org/linux-f2fs-devel/20250102101310.580277-1-guochunhai@vivo.com/
> v1->v2: Find all the discard blocks based only on discard_map in add_discard_addrs().
> ---
>   fs/f2fs/segment.c | 5 +----
>   1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index 13ee73a3c481..25ea892a42dd 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -2074,8 +2074,6 @@ static bool add_discard_addrs(struct f2fs_sb_info *sbi, struct cp_control *cpc,
>   {
>   	int entries = SIT_VBLOCK_MAP_SIZE / sizeof(unsigned long);
>   	struct seg_entry *se = get_seg_entry(sbi, cpc->trim_start);
> -	unsigned long *cur_map = (unsigned long *)se->cur_valid_map;
> -	unsigned long *ckpt_map = (unsigned long *)se->ckpt_valid_map;
>   	unsigned long *discard_map = (unsigned long *)se->discard_map;
>   	unsigned long *dmap = SIT_I(sbi)->tmp_map;
>   	unsigned int start = 0, end = -1;
> @@ -2100,8 +2098,7 @@ static bool add_discard_addrs(struct f2fs_sb_info *sbi, struct cp_control *cpc,
>   
>   	/* SIT_VBLOCK_MAP_SIZE should be multiple of sizeof(unsigned long) */
>   	for (i = 0; i < entries; i++)
> -		dmap[i] = force ? ~ckpt_map[i] & ~discard_map[i] :
> -				(cur_map[i] ^ ckpt_map[i]) & ckpt_map[i];
> +		dmap[i] = ~discard_map[i];

discard is critical, we need more sanity check here, maybe:

/* never issue discard to valid data's block address */
f2fs_bug_on(sbi, (cur_map[i] ^ discard_map[i]) & cur_map[i]);

Can you please check this?

Thanks,

>   
>   	while (force || SM_I(sbi)->dcc_info->nr_discards <=
>   				SM_I(sbi)->dcc_info->max_discards) {