Re: [PATCH v3] f2fs: fix missing discard candidates in fstrim

[Chunhai Guo]

在 3/12/2025 10:27 PM, Jaegeuk Kim 写道:
> On 03/12, 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.
> I think we need to fix the above logic.


Hi Jaegeuk,

I initially attempted to address this issue by checking whether 
seg_info_to_raw_sit() runs before add_discard_addrs(). The 
implementation can be found in the V1 patch here:
https://lore.kernel.org/linux-f2fs-devel/20250102101310.580277-1-guochunhai@xxxxxxxx/

However, after discussing with Chao, I realized that perhaps a better 
way is to find all the discard blocks based only on discard_map. This 
approach is much simpler and makes add_discard_addrs() independent of 
the pre-run of seg_info_to_raw_sit().

Could you please detail your opinion?

Thanks,


>> 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@xxxxxxxx>
>> ---
>> v2->v3: Add f2fs_bug_on() to make sure it never issues discard to valid data's block address.
>> v1->v2: Find all the discard blocks based only on discard_map in add_discard_addrs().
>> v1: https://lore.kernel.org/linux-f2fs-devel/20250102101310.580277-1-guochunhai@xxxxxxxx/
>> ---
>>   fs/f2fs/segment.c | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>> index 86e547f008f9..c8ad8e3bfebb 100644
>> --- a/fs/f2fs/segment.c
>> +++ b/fs/f2fs/segment.c
>> @@ -2075,7 +2075,6 @@ 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;
>> @@ -2097,9 +2096,10 @@ 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];
>> +	for (i = 0; i < entries; i++) {
>> +		dmap[i] = ~discard_map[i];
>> +		f2fs_bug_on(sbi, (cur_map[i] ^ discard_map[i]) & cur_map[i]);
>> +	}
>>   
>>   	while (force || SM_I(sbi)->dcc_info->nr_discards <=
>>   				SM_I(sbi)->dcc_info->max_discards) {
>> -- 
>> 2.34.1