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From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>,
        Nick Terrell <terrelln@fb.com>,
        Chris Down <chris@chrisdown.name>,
        Filipe Manana <fdmanana@suse.com>, Qu Wenruo <wqu@suse.com>,
        Nikolay Borisov <nborisov@suse.com>,
        Gabriel Niebler <gniebler@suse.com>,
        Ira Weiny <ira.weiny@intel.com>, linux-btrfs@vger.kernel.org,
        linux-kernel@vger.kernel.org, Qu Wenruo <quwenruo.btrfs@gmx.com>
Subject: Re: [RFC PATCH v2 3/3] btrfs: Use kmap_local_page() on "in_page" in zlib_compress_pages()
Date:   Sat, 18 Jun 2022 11:04:41 +0200
Message-ID: <2057523.KlZ2vcFHjT@opensuse>
In-Reply-To: <94f8d618-ec7a-f68e-c302-2639ae3d7549@gmx.com>
References: <20220617120538.18091-1-fmdefrancesco@gmail.com> <14654011.tv2OnDr8pf@opensuse> <94f8d618-ec7a-f68e-c302-2639ae3d7549@gmx.com>
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On sabato 18 giugno 2022 00:16:15 CEST Qu Wenruo wrote:
> 
> On 2022/6/18 02:13, Fabio M. De Francesco wrote:

[snip]

> 
> Thanks for pointing to the doc, and that doc is enough to answer my
> question.
> 

Well, this confirms that my changes were quite helpful :-)

[snip]

> > As I said in a recent email, I'm relatively new to kernel development,
> > especially to Btrfs and other filesystems.
> 
> That's not a big deal, that's why we're here to provide help.
> 
> >
> > However, I noted that this code does different handling depending 
> > on how many "in_page" is going to map. I am not able to say why...
> 
> AFAIK the reason is optimization.
> 
> The idea is like this, if there are multiple pages left as input, we
> copy the pages from page cache into the workspace buffer.
> 
> If there is no more than one page left, we use that page from page cache
> directly.
> 
> I believe that's the problem causing the difficult in converting to
> kmap_local_page().
> 

[snip]

> 
> I'll send out a cleanup for zlib_compress_pages(), mostly to make the
> (strm.avail_in == 0) branch to call kmap() and kunmap() in pairs,
> without holding @in_page mapped.
> 
> Would that make it easier?
> 

I was doubtful when you asked this question. However, when this morning I 
saw your patch, I soon understood that it would make that task so easy that 
a silly script could do a mechanical conversion.

Thanks so much,

Fabio