Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Sat, 19 Sep 2020 18:39:06 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Paul McKenney <paulmck@kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Will Deacon <will@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        Russell King <linux@armlinux.org.uk>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Chris Zankel <chris@zankel.net>,
        Max Filippov <jcmvbkbc@gmail.com>,
        linux-xtensa@linux-xtensa.org,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        intel-gfx <intel-gfx@lists.freedesktop.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Vineet Gupta <vgupta@synopsys.com>,
        "open list:SYNOPSYS ARC ARCHITECTURE" 
        <linux-snps-arc@lists.infradead.org>,
        Arnd Bergmann <arnd@arndb.de>, Guo Ren <guoren@kernel.org>,
        linux-csky@vger.kernel.org, Michal Simek <monstr@monstr.eu>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        linux-mips@vger.kernel.org, Nick Hu <nickhu@andestech.com>,
        Greentime Hu <green.hu@gmail.com>,
        Vincent Chen <deanbo422@gmail.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        "David S. Miller" <davem@davemloft.net>,
        linux-sparc <sparclinux@vger.kernel.org>
Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of
 kmap_atomic & friends
Message-ID: <20200919173906.GQ32101@casper.infradead.org>
References: <20200919091751.011116649@linutronix.de>
 <CAHk-=wiYGyrFRbA1cc71D2-nc5U9LM9jUJesXGqpPnB7E4X1YQ@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAHk-=wiYGyrFRbA1cc71D2-nc5U9LM9jUJesXGqpPnB7E4X1YQ@mail.gmail.com>
Precedence: bulk

On Sat, Sep 19, 2020 at 10:18:54AM -0700, Linus Torvalds wrote:
> On Sat, Sep 19, 2020 at 2:50 AM Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> > this provides a preemptible variant of kmap_atomic & related
> > interfaces. This is achieved by:
> 
> Ack. This looks really nice, even apart from the new capability.
> 
> The only thing I really reacted to is that the name doesn't make sense
> to me: "kmap_temporary()" seems a bit odd.
> 
> Particularly for an interface that really is basically meant as a
> better replacement of "kmap_atomic()" (but is perhaps also a better
> replacement for "kmap()").
> 
> I think I understand how the name came about: I think the "temporary"
> is there as a distinction from the "longterm" regular kmap(). So I
> think it makes some sense from an internal implementation angle, but I
> don't think it makes a lot of sense from an interface name.
> 
> I don't know what might be a better name, but if we want to emphasize
> that it's thread-private and a one-off, maybe "local" would be a
> better naming, and make it distinct from the "global" nature of the
> old kmap() interface?
> 
> However, another solution might be to just use this new preemptible
> "local" kmap(), and remove the old global one entirely. Yes, the old
> global one caches the page table mapping and that sounds really
> efficient and nice. But it's actually horribly horribly bad, because
> it means that we need to use locking for them. Your new "temporary"
> implementation seems to be fundamentally better locking-wise, and only
> need preemption disabling as locking (and is equally fast for the
> non-highmem case).
> 
> So I wonder if the single-page TLB flush isn't a better model, and
> whether it wouldn't be a lot simpler to just get rid of the old
> complex kmap() entirely, and replace it with this?
> 
> I agree we can't replace the kmap_atomic() version, because maybe
> people depend on the preemption disabling it also implied. But what
> about replacing the non-atomic kmap()?

My concern with that is people might use kmap() and then pass the address
to a different task.  So we need to audit the current users of kmap()
and convert any that do that into using vmap() instead.

I like kmap_local().  Or kmap_thread().