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

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Message-ID: <69fee91d-6068-4a6f-8c37-b36deb11e66a@vivo.com>
Date: Wed, 12 Mar 2025 09:38:06 +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 3/12/2025 12:03 PM, Chao Yu 写道:
>
> 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?


OK. I will sent the v3 patch.

Thanks,


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