linux-kernel - Re: [PATCH] erofs: fix the per-CPU buffer decompression for small output size

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Date:   Wed, 13 Oct 2021 19:58:26 +0800
From:   Gao Xiang <hsiangkao@...ux.alibaba.com>
To:     Yue Hu <zbestahu@...il.com>
Cc:     xiang@...nel.org, chao@...nel.org, linux-erofs@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, huyue2@...ong.com,
        zhangwen@...ong.com, zbestahu@....com
Subject: Re: [PATCH] erofs: fix the per-CPU buffer decompression for small
 output size

On Wed, Oct 13, 2021 at 07:51:55PM +0800, Gao Xiang wrote:
> Hi Yue,
> 
> On Wed, Oct 13, 2021 at 05:29:05PM +0800, Yue Hu wrote:
> > From: Yue Hu <huyue2@...ong.com>
> > 
> > Note that z_erofs_lz4_decompress() will return a positive value if
> > decompression succeeds. However, we do not copy_from_pcpubuf() due
> > to !ret. Let's fix it.
> > 
> > Signed-off-by: Yue Hu <huyue2@...ong.com>
> 
> Thanks for catching this. This is a valid issue, but it has no real
> impact to the current kernels since such pcluster in practice will be
> !inplace_io and trigger "if (nrpages_out == 1 && !rq->inplace_io) {"
> above for upstream strategies.
> 
> Our customized lz4 implementation will return 0 if success instead, so
> it has no issue to our previous products as well.
> 
> For such cases, how about updating z_erofs_lz4_decompress() to return
> 0 if success instead rather than outputsize. Since I'll return 0 if
> success for LZMA as well.

In addition, I'm fine to getting rid of such path as well, since it has
no real impact to our current upstream decompression strategy.

If it has some use cases due to decompression strategy change, we could
re-add it with some evaluation (some real numbers) later.

Thanks,
Gao Xiang

> 
> Thanks,
> Gao Xiang