linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: change maximum zstd compression buffer size

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Date:   Wed, 6 May 2020 15:45:42 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>
CC:     <kernel-team@...roid.com>, <linux-kernel@...r.kernel.org>,
        <linux-f2fs-devel@...ts.sourceforge.net>
Subject: Re: [f2fs-dev] [PATCH] f2fs: change maximum zstd compression buffer
 size

On 2020/5/6 7:05, Jaegeuk Kim wrote:
> On 05/05, Chao Yu wrote:
>> On 2020-5-4 22:30, Jaegeuk Kim wrote:
>>> From: Daeho Jeong <daehojeong@...gle.com>
>>>
>>> Current zstd compression buffer size is one page and header size less
>>> than cluster size. By this, zstd compression always succeeds even if
>>> the real compression data is failed to fit into the buffer size, and
>>> eventually reading the cluster returns I/O error with the corrupted
>>> compression data.
>>
>> What's the root cause of this issue? I didn't get it.
>>
>>>
>>> Signed-off-by: Daeho Jeong <daehojeong@...gle.com>
>>> ---
>>>  fs/f2fs/compress.c | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/fs/f2fs/compress.c b/fs/f2fs/compress.c
>>> index 4c7eaeee52336..a9fa8049b295f 100644
>>> --- a/fs/f2fs/compress.c
>>> +++ b/fs/f2fs/compress.c
>>> @@ -313,7 +313,7 @@ static int zstd_init_compress_ctx(struct compress_ctx *cc)
>>>  	cc->private = workspace;
>>>  	cc->private2 = stream;
>>>
>>> -	cc->clen = cc->rlen - PAGE_SIZE - COMPRESS_HEADER_SIZE;
>>> +	cc->clen = ZSTD_compressBound(PAGE_SIZE << cc->log_cluster_size);
>>
>> In my machine, the value is 66572 which is much larger than size of dst
>> buffer, so, in where we can tell the real size of dst buffer to zstd
>> compressor? Otherwise, if compressed data size is larger than dst buffer
>> size, when we flush compressed data into dst buffer, we may suffer overflow.
> 
> Could you give it a try compress_log_size=2 and check decompression works?

I tried some samples before submitting the patch, did you encounter app's data
corruption when using zstd algorithm? If so, let me check this issue.

Thanks,

> 
>>
>>>  	return 0;
>>>  }
>>>
>>> @@ -330,7 +330,7 @@ static int zstd_compress_pages(struct compress_ctx *cc)
>>>  	ZSTD_inBuffer inbuf;
>>>  	ZSTD_outBuffer outbuf;
>>>  	int src_size = cc->rlen;
>>> -	int dst_size = src_size - PAGE_SIZE - COMPRESS_HEADER_SIZE;
>>> +	int dst_size = cc->clen;
>>>  	int ret;
>>>
>>>  	inbuf.pos = 0;
>>>
> 
> 
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