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Message-ID: <a73d4f97-724e-4d42-9730-6ead37af25a1@vivo.com>
Date: Thu, 9 Jan 2025 10:03:19 +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 1/2] f2fs: fix missing discard candidates in fstrim
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];
}
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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