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Message-ID: <677e5946-3b3e-4f4a-892a-8aa8bfb3e45b@kernel.org>
Date: Wed, 8 Jan 2025 20:46:00 +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 2025/1/3 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?
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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