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Date:   Wed, 19 Oct 2022 20:17:31 -0700
From:   Ira Weiny <ira.weiny@intel.com>
To:     Gao Xiang <hsiangkao@linux.alibaba.com>
CC:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
        <linux-erofs@lists.ozlabs.org>, Chao Yu <chao@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] erofs: use kmap_local_page() only for erofs_bread()
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On Thu, Oct 20, 2022 at 10:18:36AM +0800, Gao Xiang wrote:
> On Wed, Oct 19, 2022 at 08:17:05PM +0200, Fabio M. De Francesco wrote:
> > On Wednesday, October 19, 2022 1:36:55 AM CEST Gao Xiang wrote:
> > > On Wed, Oct 19, 2022 at 01:21:27AM +0200, Fabio M. De Francesco wrote:
> > > > On Tuesday, October 18, 2022 11:29:21 PM CEST Gao Xiang wrote:

[snip]

> 
> That is not the simple nested unmapped case as you said above, I could take
> a very brief example:

Building on this.  The uncompressed pages always outnumber the compressed
pages, right?

> 
> 1. map a decompresed page
> 2. map a compressed page

First reverse these because you are going to need to map a new decompressed
page before another compressed page.  So:

1. map compressed
2. map decompressed

Then 4/5 and 7/8 become unmap/map new without issue.

> 3. working
> 4. decompressed page is all consumed, unmap the current decompressed page
> 5. map the next decompressed page
> 6. working
> 7. decompressed page is all consumed, unmap the current decompressed page
> 8. map the next decompressed page
> 9. working

This is more complicated but not overly so.

Simply

9.1 unmap decompressed

> 10. compressed page is all consumed, unmap the current compressed page
> 11. map the next compressed page

11.1 remap decompressed

> 12. working
> 13. ... (anyway, unmap and remap a compressed page or a decompressed page
>          in any order.)
> 
> until all process is finished.  by using kmap(), it's much simple to
> implement this, but kmap_local(), it only complexes the code.

Agreed kmap() is easier but I think this could work.

Basically you keep the compressed mapped first.

I also assume there is also a reverse of this so reverse the pages in that
case.

Thoughts?
Ira