linux-kernel - Re: [patch V2 00/18] mm/highmem: Preemptible variant of kmap

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Date:   Sat, 31 Oct 2020 00:53:02 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul McKenney <paulmck@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Ard Biesheuvel <ardb@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Christoph Hellwig <hch@....de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Vineet Gupta <vgupta@...opsys.com>,
        "open list\:SYNOPSYS ARC ARCHITECTURE" 
        <linux-snps-arc@...ts.infradead.org>,
        Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Guo Ren <guoren@...nel.org>, linux-csky@...r.kernel.org,
        Michal Simek <monstr@...str.eu>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org, Nick Hu <nickhu@...estech.com>,
        Greentime Hu <green.hu@...il.com>,
        Vincent Chen <deanbo422@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        "David S. Miller" <davem@...emloft.net>,
        linux-sparc <sparclinux@...r.kernel.org>,
        Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        linux-xtensa@...ux-xtensa.org, Matthew Wilcox <willy@...radead.org>
Subject: Re: [patch V2 00/18] mm/highmem: Preemptible variant of kmap_atomic & friends

On Sat, Oct 31 2020 at 00:26, Thomas Gleixner wrote:

> On Fri, Oct 30 2020 at 15:46, Linus Torvalds wrote:
>> On Fri, Oct 30, 2020 at 3:26 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>> To me, your patch series has two big advantages:
>>
>>  - more common code
>>
>>  - kmap_local() becomes more of a no-op
>>
>> and the last thing we want is to expand on kmap.
>
> Happy to go with that.
>
> While trying to document the mess, I just stumbled over the abuse of
> kmap_atomic_prot() in
>
> drivers/gpu/drm/ttm/ttm_bo_util.c:      dst = kmap_atomic_prot(d, prot);
> drivers/gpu/drm/ttm/ttm_bo_util.c:      src = kmap_atomic_prot(s, prot);
> drivers/gpu/drm/vmwgfx/vmwgfx_blit.c:   kmap_atomic_prot(d->dst_pages[dst_page],
> drivers/gpu/drm/vmwgfx/vmwgfx_blit.c:   kmap_atomic_prot(d->src_pages[src_page],
>
> For !HIGHMEM kmap_atomic_prot() just ignores the 'prot' argument and
> returns the page address. 
>
> Moar patches to be written ... Sigh!

Or not. This is actually correct by some definition of correct. For
the non highmem case pgprot is set via the set_memory_*() functions and
this just handles the highmem case.

Comments are overrrated...