Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends

Daniel Vetter
2020-09-19T10:38:08Z

On Sat, Sep 19, 2020 at 12:35 PM Daniel Vetter <daniel@ffwll.ch> wrote:
>
> On Sat, Sep 19, 2020 at 11:50 AM Thomas Gleixner <tglx@linutronix.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