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Message-ID: <249cf700-781b-4797-842c-fb47a48f4fce@kernel.org>
Date: Mon, 13 Jan 2025 21:32:39 +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 1/2] f2fs: fix missing discard candidates in fstrim
On 2025/1/9 18:03, Chunhai Guo wrote:
> 在 1/8/2025 8:46 PM, Chao Yu 写道:
>> On 2025/1/3 16:07, Chunhai Guo wrote:
>>> 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@...o.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.
>> My point is for fstrim case, we only need to check discard_map bitmap?
>> once bit(s) in discard_map bitmap is zero, no matter the status of
>> bit(s) in ckpt_map bitmap, we need to trigger a checkpoit for following
>> discard submission?
>
>
> Oh, yes. It is reasonable to check only the discard_map bitmap. What do
> you think about the code below?
>
> for (i = 0; i < entries; i++) {
> if (check_only)
> dmap[i] = ~discard_map[i];
> else
> dmap[i] = force ? ~ckpt_map[i] & ~discard_map[i] :
> (cur_map[i] ^ ckpt_map[i]) & ckpt_map[i];
How about this?
dmap[i] = force ? ~discard_map[i] :
(cur_map[i] ^ ckpt_map[i]) & ckpt_map[i];
Thanks,
> }
>
> Thanks,
>
>>
>> Thanks,
>>
>>> 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;
>>>>> }
>>>
>
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