linux-kernel - Re: [PATCH] f2fs: compress: remove unneed check condition

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Message-ID: <5d7fcb3c-b0a7-178c-0f5c-5b12e21cb5f0@huawei.com>
Date:   Tue, 27 Apr 2021 10:43:34 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
CC:     <linux-f2fs-devel@...ts.sourceforge.net>,
        <linux-kernel@...r.kernel.org>, <chao@...nel.org>
Subject: Re: [PATCH] f2fs: compress: remove unneed check condition

On 2021/4/27 1:05, Jaegeuk Kim wrote:
> On 04/25, Chao Yu wrote:
>> On 2021/4/25 8:47, Jaegeuk Kim wrote:
>>> On 04/22, Chao Yu wrote:
>>>> On 2021/4/22 12:04, Jaegeuk Kim wrote:
>>>>> On 04/21, Chao Yu wrote:
>>>>>> In only call path of __cluster_may_compress(), __f2fs_write_data_pages()
>>>>>> has checked SBI_POR_DOING condition, and also cluster_may_compress()
>>>>>> has checked CP_ERROR_FLAG condition, so remove redundant check condition
>>>>>> in __cluster_may_compress() for cleanup.
>>>>>
>>>>> I think cp_error can get any time without synchronization. Is it safe to say
>>>>> it's redundant?
>>>>
>>>> Yes,
>>>>
>>>> But no matter how late we check cp_error, cp_error can happen after our
>>>> check points, it won't cause regression if we remove cp_error check there,
>>>> because for compress write, it uses OPU, it won't overwrite any existed data
>>>> in device.
>>>>
>>>> Seems it will be more appropriate to check cp_error in
>>>> f2fs_write_compressed_pages() like we did in f2fs_write_single_data_page()
>>>> rather than in __cluster_may_compress().
>>>>
>>>> BTW, shouldn't we rename __cluster_may_compress() to
>>>> cluster_beyond_filesize() for better readability?
>>>
>>> f2fs_cluster_has_data()?
>>
>> Maybe cluster_has_invalid_data()? which indicates there is invalid data
>> beyond filesize.
> 
> BTW, we can compress it with zero data?

I doubt it will cause unnecessary overhead for below condition?
- write 1GB data into file
- truncate file to 0
- writeback 1GB compressed cluster contains zero data

Thanks,

> 
>>
>> Thanks,
>>
>>>
>>>>
>>>> Thanks,
>>>>
>>>>>
>>>>>>
>>>>>> Signed-off-by: Chao Yu <yuchao0@...wei.com>
>>>>>> ---
>>>>>>     fs/f2fs/compress.c | 5 -----
>>>>>>     1 file changed, 5 deletions(-)
>>>>>>
>>>>>> diff --git a/fs/f2fs/compress.c b/fs/f2fs/compress.c
>>>>>> index 3c9d797dbdd6..532c311e3a89 100644
>>>>>> --- a/fs/f2fs/compress.c
>>>>>> +++ b/fs/f2fs/compress.c
>>>>>> @@ -906,11 +906,6 @@ static bool __cluster_may_compress(struct compress_ctx *cc)
>>>>>>     		f2fs_bug_on(sbi, !page);
>>>>>> -		if (unlikely(f2fs_cp_error(sbi)))
>>>>>> -			return false;
>>>>>> -		if (unlikely(is_sbi_flag_set(sbi, SBI_POR_DOING)))
>>>>>> -			return false;
>>>>>> -
>>>>>>     		/* beyond EOF */
>>>>>>     		if (page->index >= nr_pages)
>>>>>>     			return false;
>>>>>> -- 
>>>>>> 2.29.2
>>>>> .
>>>>>
>>> .
>>>
> .
>