linux-kernel - Re: [PATCH] erofs: fix compacted_{4b_initial, 2b} when compacted_4b

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Message-ID: <YT8VvOyXIDdyD7WI@B-P7TQMD6M-0146.local>
Date:   Mon, 13 Sep 2021 17:11:24 +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 compacted_{4b_initial, 2b} when
 compacted_4b_initial > totalidx

On Mon, Sep 13, 2021 at 05:00:16PM +0800, Yue Hu wrote:
> On Mon, 13 Sep 2021 16:48:45 +0800
> Gao Xiang <hsiangkao@...ux.alibaba.com> wrote:
> 
> > Hi Yue,
> > 
> > On Mon, Sep 13, 2021 at 03:24:05PM +0800, Yue Hu wrote:
> > > From: Yue Hu <huyue2@...ong.com>
> > > 
> > > mkfs.erofs will treat compacted_4b_initial & compacted_2b as 0 if
> > > compacted_4b_initial > totalidx, kernel should be aligned with it
> > > accordingly.  
> > 
> > There is no difference between compacted_4b_initial or compacted_4b_end
> > for compacted 4B. Since in this way totalidx for compact 2B won't larger
> > than 16 (number of lclusters in a compacted 2B pack.)
> 
> However, we can see compacted_2b is a big number for this case. It should
> be pointless.

Does it has some real impact?

compacted_4b_initial is only used for the alignment use for the
first compacted_2b so that each compacted_2b pack won't cross
the block (page) boundary. And compacted_4b_end is for the last
lclusters aren't fitted in any compacted_2b pack.

If compacted_4b_initial > totalidx, I think the whole indexes
would be compacted 4B and handled in

	if (lcn < compacted_4b_initial) {
		amortizedshift = 2;
		goto out;
	}

Thanks,
Gao Xiang

> 
> Thanks.
> 
> > 
> > So it can be handled in either compacted_4b_initial or compacted_4b_end
> > cases, because there are all compacted 4B.
> > 
> > Thanks,
> > Gao Xiang
> > 
> > > 
> > > Signed-off-by: Yue Hu <huyue2@...ong.com>
> > > ---
> > >  fs/erofs/zmap.c | 5 ++++-
> > >  1 file changed, 4 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/erofs/zmap.c b/fs/erofs/zmap.c
> > > index 9fb98d8..4f941b6 100644
> > > --- a/fs/erofs/zmap.c
> > > +++ b/fs/erofs/zmap.c
> > > @@ -369,7 +369,10 @@ static int compacted_load_cluster_from_disk(struct z_erofs_maprecorder *m,
> > >  	if (compacted_4b_initial == 32 / 4)
> > >  		compacted_4b_initial = 0;
> > >  
> > > -	if (vi->z_advise & Z_EROFS_ADVISE_COMPACTED_2B)
> > > +	if (compacted_4b_initial > totalidx) {
> > > +		compacted_4b_initial = 0;
> > > +		compacted_2b = 0;
> > > +	} else if (vi->z_advise & Z_EROFS_ADVISE_COMPACTED_2B)
> > >  		compacted_2b = rounddown(totalidx - compacted_4b_initial, 16);
> > >  	else
> > >  		compacted_2b = 0;
> > > -- 
> > > 1.9.1