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Date:   Sat, 9 May 2020 09:56:31 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Gao Xiang <hsiangkao@....com>
CC:     <jaegeuk@...nel.org>, <linux-kernel@...r.kernel.org>,
        <linux-f2fs-devel@...ts.sourceforge.net>, <chao@...nel.org>
Subject: Re: [f2fs-dev] [PATCH] f2fs: compress: allow lz4 to compress data
 partially

Hi Xiang,

On 2020/5/8 18:23, Gao Xiang wrote:
> Hi Chao,
> 
> On Fri, May 08, 2020 at 05:47:09PM +0800, Chao Yu wrote:
>> For lz4 worst compress case, caller should allocate buffer with size
>> of LZ4_compressBound(inputsize) for target compressed data storing.
>>
>> However lz4 supports partial data compression, so we can get rid of
>> output buffer size limitation now, then we can avoid 2 * 4KB size
>> intermediate buffer allocation when log_cluster_size is 2, and avoid
>> unnecessary compressing work of compressor if we can not save at
>> least 4KB space.
>>
>> Suggested-by: Daeho Jeong <daehojeong@...gle.com>
>> Signed-off-by: Chao Yu <yuchao0@...wei.com>
>> ---
>>  fs/f2fs/compress.c | 15 +++++++++------
>>  1 file changed, 9 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/f2fs/compress.c b/fs/f2fs/compress.c
>> index 5e4947250262..23825f559bcf 100644
>> --- a/fs/f2fs/compress.c
>> +++ b/fs/f2fs/compress.c
>> @@ -228,7 +228,12 @@ static int lz4_init_compress_ctx(struct compress_ctx *cc)
>>  	if (!cc->private)
>>  		return -ENOMEM;
>>  
>> -	cc->clen = LZ4_compressBound(PAGE_SIZE << cc->log_cluster_size);
>> +	/*
>> +	 * we do not change cc->clen to LZ4_compressBound(inputsize) to
>> +	 * adapt worst compress case, because lz4 algorithm supports partial
>> +	 * compression.
> 
> Actually, in this case the lz4 compressed block is not valid (at least not ended
> in a valid lz4 EOF), and AFAIK the current public lz4 API cannot keep on
> compressing this block. so IMO "partial compression" for an invalid lz4
> block may be confusing.

Yes, comments could be improved to avoid confusing.

> 
> I think some words like "because lz4 implementation can handle output buffer
> budget properly" or similar stuff could be better.

It's better, let me change to use this, thanks for the advice.

Thanks,

> 
> The same to the patch title and the commit message.
> 
> Thanks,
> Gao Xiang
> 
> 
>> +	 */
>> +	cc->clen = cc->rlen - PAGE_SIZE - COMPRESS_HEADER_SIZE;
>>  	return 0;
>>  }
>>  
>> @@ -244,11 +249,9 @@ static int lz4_compress_pages(struct compress_ctx *cc)
>>  
>>  	len = LZ4_compress_default(cc->rbuf, cc->cbuf->cdata, cc->rlen,
>>  						cc->clen, cc->private);
>> -	if (!len) {
>> -		printk_ratelimited("%sF2FS-fs (%s): lz4 compress failed\n",
>> -				KERN_ERR, F2FS_I_SB(cc->inode)->sb->s_id);
>> -		return -EIO;
>> -	}
>> +	if (!len)
>> +		return -EAGAIN;
>> +
>>  	cc->clen = len;
>>  	return 0;
>>  }
>> -- 
>> 2.18.0.rc1
> .
> 

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