Re: [f2fs-dev] 答复: [PATCH] f2fs: compress: avoid unnecessary check in f2fs_prepare_compress_overwrite

[Chao Yu <yuchao0@huawei.com>]

Hi Xiang,

On 2021/5/6 17:58, Gao Xiang wrote:
> Hi Chao,
> 
> On Thu, May 06, 2021 at 05:15:04PM +0800, Chao Yu wrote:
>> On 2021/4/26 17:00, Gao Xiang wrote:
>>> On Mon, Apr 26, 2021 at 04:42:20PM +0800, changfengnan@vivo.com wrote:
>>>> Thank you for the reminder, I hadn't thought about fallocate before.
>>>> I have done some tests and the results are as expected.
>>>> Here is my test method, create a compressed file, and use fallocate with keep size, when write data to expand area, f2fs_prepare_compress_overwrite
>>>> always return 0, the behavior is same as my patch , apply my patch can avoid those check.
>>>> Is there anything else I haven't thought of?
>>>
>>> Nope, I didn't look into the implementation. Just a wild guess.
>>>
>>> (I just wondered if the cluster size is somewhat large (e.g. 64k),
>>>    but with a partial fallocate (e.g. 16k), and does it behave ok?
>>>    or some other corner cases/conditions are needed.)
>>
>> Xiang, sorry for late reply.
>>
>> Now, f2fs triggers compression only if one cluster is fully written,
>> e.g. cluster size is 16kb, isize is 8kb, then the first cluster is
>> non-compressed one, so we don't need to prepare for compressed
>> cluster overwrite during write_begin(). Also, blocks fallocated
>> beyond isize should never be compressed, so we don't need to worry
>> about that.
>>
> 
> Yeah, that could make it unnoticable. but my main concern is actually
> not what the current runtime compression logic is, but what the on-disk
> compresion format actually is, or there could cause compatibility
> issue if some later kernel makes full use of this and use old kernels

That's related, if there is layout v2 or we updated runtime compression
policy later, it needs to reconsider newly introduced logic of this patch,
I guess we need to add comments here to indicate why we can skip the
preparation function.

> instead (also considering some corrupted compression indexes, which
> is not generated by the normal runtime compression logic.)

Yes, that's good concern, but that was not done by
f2fs_prepare_compress_overwrite(), another sanity check logic needs
to be designed and implemented in separated patch.

> 
> My own suggestion about this is still verifying compress indexes
> first rather than relying much on runtime logic constraint. (Except
> that this patch can show signifiant benefit performance numbers to
> prove it can improve performance a lot.) Just my own premature
> thoughts.

Fengnan, could you please give some numbers to show how that check can
impact performance?

Thanks,

> 
> Thanks,
> Gao Xiang
> 
>> Thanks,
>>
>>>
>>> If that is fine, I have no problem about this, yet i_size here
>>> is generally somewhat risky since after post-EOF behavior
>>> changes (e.g. supporting FALLOC_FL_ZERO_RANGE with keep size
>>> later), it may cause some potential regression.
>>>
>>>>
>>>> -----邮件原件-----
>>>> 发件人: Gao Xiang <hsiangkao@redhat.com>
>>>> 发送时间: 2021年4月26日 11:19
>>>> 收件人: Fengnan Chang <changfengnan@vivo.com>
>>>> 抄送: chao@kernel.org; jaegeuk@kernel.org;
>>>> linux-f2fs-devel@lists.sourceforge.net
>>>> 主题: Re: [f2fs-dev] [PATCH] f2fs: compress: avoid unnecessary check in
>>>> f2fs_prepare_compress_overwrite
>>>>
>>>> On Mon, Apr 26, 2021 at 10:11:53AM +0800, Fengnan Chang wrote:
>>>>> when write compressed file with O_TRUNC, there will be a lot of
>>>>> unnecessary check valid blocks in f2fs_prepare_compress_overwrite,
>>>>> especially when written in page size, remove it.
>>>>>
>>>>> Signed-off-by: Fengnan Chang <changfengnan@vivo.com>
>>>>
>>>> Even though I didn't look into the whole thing, my reaction here is roughly
>>>> how to handle fallocate with keep size? Does it work as expected?
>>>>
>>>>> ---
>>>>>    fs/f2fs/data.c | 4 ++++
>>>>>    1 file changed, 4 insertions(+)
>>>>>
>>>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c index
>>>>> cf935474ffba..9c3b0849f35e 100644
>>>>> --- a/fs/f2fs/data.c
>>>>> +++ b/fs/f2fs/data.c
>>>>> @@ -3270,6 +3270,7 @@ static int f2fs_write_begin(struct file *file,
>>>>> struct address_space *mapping,
>>>>>    	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
>>>>>    	struct page *page = NULL;
>>>>>    	pgoff_t index = ((unsigned long long) pos) >> PAGE_SHIFT;
>>>>> +	pgoff_t end = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
>>>>>    	bool need_balance = false, drop_atomic = false;
>>>>>    	block_t blkaddr = NULL_ADDR;
>>>>>    	int err = 0;
>>>>> @@ -3306,6 +3307,9 @@ static int f2fs_write_begin(struct file *file,
>>>>> struct address_space *mapping,
>>>>>
>>>>>    		*fsdata = NULL;
>>>>>
>>>>> +		if (index >= end)
>>>>> +			goto repeat;
>>>>> +
>>>>>    		ret = f2fs_prepare_compress_overwrite(inode, pagep,
>>>>>    							index, fsdata);
>>>>>    		if (ret < 0) {
>>>>> --
>>>>> 2.29.0
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>>
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>>>
>>
>>
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> .
> 


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