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

[Chao Yu]

On 2020/5/6 22:56, Jaegeuk Kim wrote:
> On 05/06, Chao Yu wrote:
>> 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@xxxxxxxxxx>
>>>>>
>>>>> 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@xxxxxxxxxx>
>>>>> ---
>>>>>  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.
> 
> Daeho reported:
> 1. cp -a src_file comp_dir/dst_file (comp_dir is a directory for compression)
> 2. sync comp_dir/dst_file
> 3. echo 3 > /proc/sys/vm/drop_caches
> 4. cat comp_dir/dst_file > dump (it returns I/O error depending on the file).

 * ZSTD_endStream() to complete the flush. It returns the number of bytes left
 * in the internal buffer and must be called until it returns 0.

It looks we need to check return value of ZSTD_endStream() to see whether
dst buffer has enough space to store all compressed data.

Thanks,

> 
>>
>> 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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>>> .
>>>
> .
>