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Date:   Sun, 11 Oct 2020 22:28:18 -0700
From:   Ira Weiny <>
To:     Coly Li <>
Cc:     Andrew Morton <>,
        Thomas Gleixner <>,
        Ingo Molnar <>, Borislav Petkov <>,
        Andy Lutomirski <>,
        Peter Zijlstra <>,
        Kent Overstreet <>,,
        Dave Hansen <>,
        Dan Williams <>,
        Fenghua Yu <>,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
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, wrote:
> > From: Ira Weiny <>
> > 
> > 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.
> Coly Li

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