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

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Message-ID: <381ac49a-25e9-4c37-9855-e2adb64fa81d@kernel.org>
Date: Wed, 12 Mar 2025 12:03:04 +0800
From: Chao Yu <chao@...nel.org>
To: Chunhai Guo <guochunhai@...o.com>, "jaegeuk@...nel.org"
 <jaegeuk@...nel.org>
Cc: chao@...nel.org,
 "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



On 3/12/25 11:19, Chunhai Guo wrote:
> 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, so it looks fine now, can you please update the patch w/ above
f2fs_bug_on check?

Thanks,

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