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

[Chao Yu]

On 2020-5-5 9:52, 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);

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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