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Date:   Mon, 13 Sep 2021 19:19:16 +0800
From:   Gao Xiang <hsiangkao@linux.alibaba.com>
To:     Yue Hu <zbestahu@163.com>
Cc:     Yue Hu <zbestahu@gmail.com>, xiang@kernel.org, chao@kernel.org,
        linux-erofs@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        huyue2@yulong.com, zhangwen@yulong.com
Subject: Re: [PATCH] erofs: fix compacted_{4b_initial, 2b} when
 compacted_4b_initial > totalidx
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Hi Yue,

On Mon, Sep 13, 2021 at 06:58:36PM +0800, Yue Hu wrote:
> Hi Xiang,
> 
> On Mon, 13 Sep 2021 17:11:24 +0800
> Gao Xiang <hsiangkao@linux.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@linux.alibaba.com> wrote:
> > >   
> > > > Hi Yue,
> > > > 
> > > > On Mon, Sep 13, 2021 at 03:24:05PM +0800, Yue Hu wrote:  
> > > > > From: Yue Hu <huyue2@yulong.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.

Okay, make sense. If the number of compacted_2b misleads, how about
just

	if ((vi->z_advise & Z_EROFS_ADVISE_COMPACTED_2B) &&
	    compacted_4b_initial <= totalidx) {
		compacted_2b = ...;
	} else {
		compacted_2b = 0;
	}

, and refine the commit message to point out the following facts for
other folks:

- compacted_4b_initial is used contain the very first lclusters in order
  to fulfill the alignment of the first compacted_2b pack;

- compacted_4b_end is used for the last lclusters which aren't fitted in
  the previous compacted_2b packs;

- if compacted_4b_initial > totalidx, the whole indexes will be compacted
  4B and handled with compacted_4b_initial.

Thanks,
Gao Xiang

> 
> Thanks.