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

[Chao Yu <chao@kernel.org>]

On 2020-5-5 9:52, Chao Yu wrote:
> On 2020-5-4 22:30, Jaegeuk Kim wrote:
>> From: Daeho Jeong <daehojeong@google.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@google.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);

Max size of intermediate buffer has been set as below?

- zstd_init_compress_ctx
  - params = ZSTD_getParams(F2FS_ZSTD_DEFAULT_CLEVEL, cc->rlen, 0)
  - ZSTD_CStreamWorkspaceBound(params.cParams)
   - outBuffSize = ZSTD_compressBound(blockSize) + 1;
   - workspace_size = ... + outBuffSize + ...
  - workspace = f2fs_kvmalloc(workspace_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.
>
>>      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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