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

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Date:   Tue, 5 May 2020 16:05:59 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <chao@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, kernel-team@...roid.com
Subject: Re: [f2fs-dev] [PATCH] f2fs: change maximum zstd compression buffer
 size

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?

> 
> >  	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;
> >