linux-kernel - Re: [PATCH RFC PKS/PMEM 48/58] drivers/md: Utilize new kmap

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Message-ID: <20201012052817.GZ2046448@iweiny-DESK2.sc.intel.com>
Date:   Sun, 11 Oct 2020 22:28:18 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Coly Li <colyli@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kent Overstreet <kent.overstreet@...il.com>, x86@...nel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
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Subject: Re: [PATCH RFC PKS/PMEM 48/58] drivers/md: Utilize new kmap_thread()

On Sat, Oct 10, 2020 at 10:20:34AM +0800, Coly Li wrote:
> On 2020/10/10 03:50, ira.weiny@...el.com wrote:
> > From: Ira Weiny <ira.weiny@...el.com>
> > 
> > These kmap() calls are localized to a single thread.  To avoid the over
> > head of global PKRS updates use the new kmap_thread() call.
> > 
> 
> Hi Ira,
> 
> There were a number of options considered.
> 
> 1) Attempt to change all the thread local kmap() calls to kmap_atomic()
> 2) Introduce a flags parameter to kmap() to indicate if the mapping
> should be global or not
> 3) Change ~20-30 call sites to 'kmap_global()' to indicate that they
> require a global mapping of the pages
> 4) Change ~209 call sites to 'kmap_thread()' to indicate that the
> mapping is to be used within that thread of execution only
> 
> 
> I copied the above information from patch 00/58 to this message. The
> idea behind kmap_thread() is fine to me, but as you said the new api is
> very easy to be missed in new code (even for me). I would like to be
> supportive to option 2) introduce a flag to kmap(), then we won't forget
> the new thread-localized kmap method, and people won't ask why a
> _thread() function is called but no kthread created.

Thanks for the feedback.

I'm going to hold off making any changes until others weigh in.  FWIW, I kind
of like option 2 as well.  But there is already kmap_atomic() so it seemed like
kmap_XXXX() was more in line with the current API.

Thanks,
Ira

> 
> Thanks.
> 
> 
> Coly Li
>