Re: [f2fs-dev] [PATCH v2] f2fs: update dirty status for CURSEG as well

[Chao Yu <chao@kernel.org>]

Hi Yunlong,

I think you're so busy, I just help to refactor your patch, and send it out
authored with you, please check that patch, if you have different opinion, let
me know.

Thanks,

On 2017/10/16 11:43, Chao Yu wrote:
> On 2017/10/14 20:53, Yunlong Song wrote:
>> Oh, yes it is. I found that problem in a kernel tree which does not have 
>> commit
>> c6f82fe90d7458e5fa190a6820bfc24f96b0de4e (Revert "f2fs: put allocate_segment
>> after refresh_sit_entry"). In that kernel, the allocate_segment is still 
>> behind
>> refresh_sit_entry. Now I understand the commit message:
>> "This makes a leak to register dirty segments. I reproduced the issue by
>>      modified postmark which injects a lot of file create/delete/update and
>>      finally triggers huge number of SSR allocations."
>>
>> The reason is that if refresh_sit_entry is before allocate_segment, then the
>> dirty status of CURSEG is not updated, as a result, the count of dirty 
>> segments
>> is wrong, which is much smaller than its real value. Then the f2fs_gc 
>> can not
>> do its work since it can not even get one victim, then the free segments are
>> used up and then triggers much SSR. So Jay reverts the patch.
>>
>> It seems there are two options:
>> (1) keep this patch ([PATCH v2] f2fs: update dirty status for CURSEG as 
>> well)
>> and we can recover commit 3436c4bdb30de421d46f58c9174669fbcfd40ce0
>> (f2fs: put allocate_segment after refresh_sit_entry)
>> (2) remove this patch at all
>>
>> It seems (1) is robust, but (2) avoids unnecessary check.
> 
> What about reverting 5e443818fa0b ("f2fs: handle dirty segments inside
> refresh_sit_entry") to keep the original order:
> 
> 1. update sit info
> 2. allocate new segment
> 3. update dirty status of segment
> 
> Thanks,
> 
>>
>> On 2017/10/14 8:14, Chao Yu wrote:
>>> On 2017/10/13 21:21, Yunlong Song wrote:
>>>> Without this patch, it will cause all the free segments using up in some
>>>> corner case. For example, there are 100 segments, and 20 of them are
>>>> reserved for ovp. If 79 segments are full of data, segment 80 becomes
>>>> CURSEG segment, write 512 blocks and then delete 511 blocks. Since it is
>>>> CURSEG segment, the __locate_dirty_segment will not update its dirty
>>>> status. Then the dirty_segments(sbi) is 0, f2fs_gc will fail to
>>>> get_victim, and f2fs_balance_fs will fail to trigger gc action. After
>>>> f2fs_balance_fs returns, f2fs can continue to write data to segment 81.
>>>> Again, segment 81 becomes CURSEG segment, write 512 blocks and delete
>>>> 511 blocks, the dirty_segments(sbi) is 0 and f2fs_gc fail again. This
>>>> can finally use up all the free segments and cause panic.
>>> Look into this patch again, I found refresh_sit_entry is called after
>>> ->allocate_segment, so if all 512 blocks were allocated, log header should
>>> have been moved to another segment, so locate_dirty_segment in
>>> refresh_sit_entry should update dirty status of previous segment correctly,
>>> anything I'm missing?
>>>
>>> Thanks,
>>>
>>>> Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
>>>> ---
>>>>   fs/f2fs/segment.c | 4 ++--
>>>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>>>> index bfbcff8..0fce076 100644
>>>> --- a/fs/f2fs/segment.c
>>>> +++ b/fs/f2fs/segment.c
>>>> @@ -687,7 +687,7 @@ static void __locate_dirty_segment(struct f2fs_sb_info *sbi, unsigned int segno,
>>>>   	struct dirty_seglist_info *dirty_i = DIRTY_I(sbi);
>>>>   
>>>>   	/* need not be added */
>>>> -	if (IS_CURSEG(sbi, segno))
>>>> +	if (IS_CURSEG(sbi, segno) && dirty_type == PRE)
>>>>   		return;
>>>>   
>>>>   	if (!test_and_set_bit(segno, dirty_i->dirty_segmap[dirty_type]))
>>>> @@ -737,7 +737,7 @@ static void locate_dirty_segment(struct f2fs_sb_info *sbi, unsigned int segno)
>>>>   	struct dirty_seglist_info *dirty_i = DIRTY_I(sbi);
>>>>   	unsigned short valid_blocks;
>>>>   
>>>> -	if (segno == NULL_SEGNO || IS_CURSEG(sbi, segno))
>>>> +	if (segno == NULL_SEGNO)
>>>>   		return;
>>>>   
>>>>   	mutex_lock(&dirty_i->seglist_lock);
>>>>
>>> .
>>>
>>
>