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

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Message-ID: <20210913185836.088e7059.zbestahu@163.com>
Date:   Mon, 13 Sep 2021 18:58:36 +0800
From:   Yue Hu <zbestahu@....com>
To:     Gao Xiang <hsiangkao@...ux.alibaba.com>
Cc:     Yue Hu <zbestahu@...il.com>, xiang@...nel.org, chao@...nel.org,
        linux-erofs@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        huyue2@...ong.com, zhangwen@...ong.com
Subject: Re: [PATCH] erofs: fix compacted_{4b_initial, 2b} when
 compacted_4b_initial > totalidx

Hi Xiang,

On Mon, 13 Sep 2021 17:11:24 +0800
Gao Xiang <hsiangkao@...ux.alibaba.com> wrote:

> 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?

No real impact to correct result.

> 
> 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;
> 	}

Yes, it is. 

My point is why we need compacted_2b here for this case. If it's
not helpful/used for next code logic, we should remove/avoid it.
I think that may cause some misunderstanding and consume unneeded
CPU resources.

Thanks.

> 
> 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