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Message-ID: <CAKMK7uENFDANQKebS_H0bhHeQRijrp1aVHQqyZPute3KBZ+fVQ@mail.gmail.com>
Date: Sat, 19 Sep 2020 12:37:53 +0200
From: Daniel Vetter <daniel@...ll.ch>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
"open list:GENERIC INCLUDE/A..." <linux-arch@...r.kernel.org>,
Linus Torvalds <torvalds@...uxfoundation.org>,
Paul McKenney <paulmck@...nel.org>, X86 ML <x86@...nel.org>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Peter Zijlstra <peterz@...radead.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>,
Will Deacon <will@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Linux-MM <linux-mm@...ck.org>,
Russell King <linux@...linux.org.uk>,
Linux ARM <linux-arm-kernel@...ts.infradead.org>,
Chris Zankel <chris@...kel.net>,
Max Filippov <jcmvbkbc@...il.com>,
linux-xtensa@...ux-xtensa.org,
Jani Nikula <jani.nikula@...ux.intel.com>,
Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
Rodrigo Vivi <rodrigo.vivi@...el.com>,
David Airlie <airlied@...ux.ie>,
intel-gfx <intel-gfx@...ts.freedesktop.org>,
dri-devel <dri-devel@...ts.freedesktop.org>,
Ard Biesheuvel <ardb@...nel.org>,
Herbert Xu <herbert@...dor.apana.org.au>,
Vineet Gupta <vgupta@...opsys.com>,
arcml <linux-snps-arc@...ts.infradead.org>,
Arnd Bergmann <arnd@...db.de>, 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>, sparclinux@...r.kernel.org
Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of
kmap_atomic & friends
On Sat, Sep 19, 2020 at 12:35 PM Daniel Vetter <daniel@...ll.ch> wrote:
>
> On Sat, Sep 19, 2020 at 11:50 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > First of all, sorry for the horribly big Cc list!
> >
> > Following up to the discussion in:
> >
> > https://lore.kernel.org/r/20200914204209.256266093@linutronix.de
> >
> > this provides a preemptible variant of kmap_atomic & related
> > interfaces. This is achieved by:
> >
> > - Consolidating all kmap atomic implementations in generic code
> >
> > - Switching from per CPU storage of the kmap index to a per task storage
> >
> > - Adding a pteval array to the per task storage which contains the ptevals
> > of the currently active temporary kmaps
> >
> > - Adding context switch code which checks whether the outgoing or the
> > incoming task has active temporary kmaps. If so, the outgoing task's
> > kmaps are removed and the incoming task's kmaps are restored.
> >
> > - Adding new interfaces k[un]map_temporary*() which are not disabling
> > preemption and can be called from any context (except NMI).
> >
> > Contrary to kmap() which provides preemptible and "persistant" mappings,
> > these interfaces are meant to replace the temporary mappings provided by
> > kmap_atomic*() today.
> >
> > This allows to get rid of conditional mapping choices and allows to have
> > preemptible short term mappings on 64bit which are today enforced to be
> > non-preemptible due to the highmem constraints. It clearly puts overhead on
> > the highmem users, but highmem is slow anyway.
> >
> > This is not a wholesale conversion which makes kmap_atomic magically
> > preemptible because there might be usage sites which rely on the implicit
> > preempt disable. So this needs to be done on a case by case basis and the
> > call sites converted to kmap_temporary.
> >
> > Note, that this is only lightly tested on X86 and completely untested on
> > all other architectures.
> >
> > The lot is also available from
> >
> > git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git highmem
>
> I think it should be the case, but I want to double check: Will
> copy_*_user be allowed within a kmap_temporary section? This would
> allow us to ditch an absolute pile of slowpaths.
(coffee just kicked in) copy_*_user is ofc allowed, but if you hit a
page fault you get a short read/write. This looks like it would remove
the need to handle these in a slowpath, since page faults can now be
served in this new kmap_temporary sections. But this sounds too good
to be true, so I'm wondering what I'm missing.
-Daniel
>
> >
> > Thanks,
> >
> > tglx
> > ---
> > a/arch/arm/mm/highmem.c | 121 ---------------------
> > a/arch/microblaze/mm/highmem.c | 78 -------------
> > a/arch/nds32/mm/highmem.c | 48 --------
> > a/arch/powerpc/mm/highmem.c | 67 -----------
> > a/arch/sparc/mm/highmem.c | 115 --------------------
> > arch/arc/Kconfig | 1
> > arch/arc/include/asm/highmem.h | 8 +
> > arch/arc/mm/highmem.c | 44 -------
> > arch/arm/Kconfig | 1
> > arch/arm/include/asm/highmem.h | 30 +++--
> > arch/arm/mm/Makefile | 1
> > arch/csky/Kconfig | 1
> > arch/csky/include/asm/highmem.h | 4
> > arch/csky/mm/highmem.c | 75 -------------
> > arch/microblaze/Kconfig | 1
> > arch/microblaze/include/asm/highmem.h | 6 -
> > arch/microblaze/mm/Makefile | 1
> > arch/microblaze/mm/init.c | 6 -
> > arch/mips/Kconfig | 1
> > arch/mips/include/asm/highmem.h | 4
> > arch/mips/mm/highmem.c | 77 -------------
> > arch/mips/mm/init.c | 3
> > arch/nds32/Kconfig.cpu | 1
> > arch/nds32/include/asm/highmem.h | 21 ++-
> > arch/nds32/mm/Makefile | 1
> > arch/powerpc/Kconfig | 1
> > arch/powerpc/include/asm/highmem.h | 6 -
> > arch/powerpc/mm/Makefile | 1
> > arch/powerpc/mm/mem.c | 7 -
> > arch/sparc/Kconfig | 1
> > arch/sparc/include/asm/highmem.h | 7 -
> > arch/sparc/mm/Makefile | 3
> > arch/sparc/mm/srmmu.c | 2
> > arch/x86/include/asm/fixmap.h | 1
> > arch/x86/include/asm/highmem.h | 12 +-
> > arch/x86/include/asm/iomap.h | 29 +++--
> > arch/x86/mm/highmem_32.c | 59 ----------
> > arch/x86/mm/init_32.c | 15 --
> > arch/x86/mm/iomap_32.c | 57 ----------
> > arch/xtensa/Kconfig | 1
> > arch/xtensa/include/asm/highmem.h | 9 +
> > arch/xtensa/mm/highmem.c | 44 -------
> > b/arch/x86/Kconfig | 3
> > include/linux/highmem.h | 141 +++++++++++++++---------
> > include/linux/io-mapping.h | 2
> > include/linux/sched.h | 9 +
> > kernel/sched/core.c | 10 +
> > mm/Kconfig | 3
> > mm/highmem.c | 192 ++++++++++++++++++++++++++++++++--
> > 49 files changed, 422 insertions(+), 909 deletions(-)
>
>
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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