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

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Message-ID: <74811428-9fee-467b-802f-dcb9f9dbb5a5@vivo.com>
Date: Wed, 12 Mar 2025 03:19:25 +0000
From: Chunhai Guo <guochunhai@...o.com>
To: Chao Yu <chao@...nel.org>, Chunhai Guo <guochunhai@...o.com>,
	"jaegeuk@...nel.org" <jaegeuk@...nel.org>
CC: "linux-f2fs-devel@...ts.sourceforge.net"
	<linux-f2fs-devel@...ts.sourceforge.net>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] f2fs: fix missing discard candidates in fstrim

在 1/20/2025 7:45 PM, Chao Yu 写道:
> 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?

Sure. I have added the BUG_ON check and performed the following tests
without issue:
1. Ran xfstests and fsstress in the QEMU environment.

2. Ran monkey and reboot tests on ARM64 Android devices with the 6.6 kernel.

Thanks,


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