Re: [PATCH 1/2] f2fs: fix missing discard candidates in fstrim

[Chunhai Guo <guochunhai@vivo.com>]

在 1/3/2025 11:26 AM, Chao Yu 写道:
> On 2025/1/2 18:13, Chunhai Guo wrote:
>> fstrim may miss candidates that need to be discarded in fstrim, 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 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(), which results in failure.
> Chunhai,
>
> Can you please use nodiscard option due to fstrim stopped to return
> trimmed length after below commit:
>
> 5a6154920faf ("f2fs: don't issue discard commands in online discard is on")

Thank you for your explanation, but I guess this issue is not relevant 
to this commit, and I understand that '0 B (0 bytes) trimmed' is fine.

The real problem is that there are actually many candidates that should 
be handled in fstrim, but it cannot find any of them.

f2fs_trim_fs()
     f2fs_write_checkpoint()
         ...
         if (cpc->reason & CP_DISCARD) {
                 if (!f2fs_exist_trim_candidates(sbi, cpc)) {
                     unblock_operations(sbi);
                     goto out; // Not candidate is found here and exit.
                 }
             ...
         }

>> 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
>>
>> [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@vivo.com>
>> ---
>>    fs/f2fs/segment.c | 10 +++++-----
>>    1 file changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>> index eade36c5ef13..8fe9f794b581 100644
>> --- a/fs/f2fs/segment.c
>> +++ b/fs/f2fs/segment.c
>> @@ -2070,7 +2070,7 @@ static int f2fs_issue_discard(struct f2fs_sb_info *sbi,
>>    }
>>    
>>    static bool add_discard_addrs(struct f2fs_sb_info *sbi, struct cp_control *cpc,
>> -							bool check_only)
>> +					bool synced, bool check_only)
>>    {
>>    	int entries = SIT_VBLOCK_MAP_SIZE / sizeof(unsigned long);
>>    	struct seg_entry *se = get_seg_entry(sbi, cpc->trim_start);
>> @@ -2098,7 +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] :
>> +		dmap[i] = synced ? ~ckpt_map[i] & ~discard_map[i] :
> I guess this condition "force ? ~ckpt_map[i] & ~discard_map[i]" didn't cover
> all below cases, thoughts?
> - ckpt_map[i] == 0 // write data, and then remove data before checkpoint
> - ckpt_map[i] != 0 // remove data existed in previous checkpoint

 From the handling of ckpt_valid_map in update_sit_entry(), I guess the 
condition can cover both cases.
For example, when the checkpoint is enabled, all the set bits in the 
ckpt_valid_map remain set before the checkpoint (even when the blocks 
are deleted), which makes it find all the right bits in both cases.

Thanks,

>
> Thanks,
>
>>    				(cur_map[i] ^ ckpt_map[i]) & ckpt_map[i];
>>    
>>    	while (force || SM_I(sbi)->dcc_info->nr_discards <=
>> @@ -3275,7 +3275,7 @@ bool f2fs_exist_trim_candidates(struct f2fs_sb_info *sbi,
>>    
>>    	down_write(&SIT_I(sbi)->sentry_lock);
>>    	for (; cpc->trim_start <= cpc->trim_end; cpc->trim_start++) {
>> -		if (add_discard_addrs(sbi, cpc, true)) {
>> +		if (add_discard_addrs(sbi, cpc, false, true)) {
>>    			has_candidate = true;
>>    			break;
>>    		}
>> @@ -4611,7 +4611,7 @@ void f2fs_flush_sit_entries(struct f2fs_sb_info *sbi, struct cp_control *cpc)
>>    			/* add discard candidates */
>>    			if (!(cpc->reason & CP_DISCARD)) {
>>    				cpc->trim_start = segno;
>> -				add_discard_addrs(sbi, cpc, false);
>> +				add_discard_addrs(sbi, cpc, false, false);
>>    			}
>>    
>>    			if (to_journal) {
>> @@ -4653,7 +4653,7 @@ void f2fs_flush_sit_entries(struct f2fs_sb_info *sbi, struct cp_control *cpc)
>>    		__u64 trim_start = cpc->trim_start;
>>    
>>    		for (; cpc->trim_start <= cpc->trim_end; cpc->trim_start++)
>> -			add_discard_addrs(sbi, cpc, false);
>> +			add_discard_addrs(sbi, cpc, true, false);
>>    
>>    		cpc->trim_start = trim_start;
>>    	}