linux-kernel - Re: [PATCH v4 0/4] arm64: drop pfn_valid_within() and simplify pfn

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Date:   Wed, 12 May 2021 09:59:33 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        David Hildenbrand <david@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>,
        kvmarm <kvmarm@...ts.cs.columbia.edu>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH v4 0/4] arm64: drop pfn_valid_within() and simplify pfn_valid()

On Wed, 12 May 2021 at 09:34, Mike Rapoport <rppt@...ux.ibm.com> wrote:
>
> On Wed, May 12, 2021 at 09:00:02AM +0200, Ard Biesheuvel wrote:
> > On Tue, 11 May 2021 at 12:05, Mike Rapoport <rppt@...nel.org> wrote:
> > >
> > > From: Mike Rapoport <rppt@...ux.ibm.com>
> > >
> > > Hi,
> > >
> > > These patches aim to remove CONFIG_HOLES_IN_ZONE and essentially hardwire
> > > pfn_valid_within() to 1.
> > >
> > > The idea is to mark NOMAP pages as reserved in the memory map and restore
> > > the intended semantics of pfn_valid() to designate availability of struct
> > > page for a pfn.
> > >
> > > With this the core mm will be able to cope with the fact that it cannot use
> > > NOMAP pages and the holes created by NOMAP ranges within MAX_ORDER blocks
> > > will be treated correctly even without the need for pfn_valid_within.
> > >
> > > The patches are boot tested on qemu-system-aarch64.
> > >
> >
> > Did you use EFI boot when testing this? The memory map is much more
> > fragmented in that case, so this would be a good data point.
>
> Right, something like this:
>

Yes, although it is not always that bad.

> [    0.000000] Early memory node ranges
> [    0.000000]   node   0: [mem 0x0000000040000000-0x00000000ffffbfff]
> [    0.000000]   node   0: [mem 0x00000000ffffc000-0x00000000ffffffff]

This is allocated below 4 GB by the firmware, for reasons that are
only valid on x86 (where some of the early boot chain is IA32 only)

> [    0.000000]   node   0: [mem 0x0000000100000000-0x00000004386fffff]
> [    0.000000]   node   0: [mem 0x0000000438700000-0x000000043899ffff]
> [    0.000000]   node   0: [mem 0x00000004389a0000-0x00000004389bffff]
> [    0.000000]   node   0: [mem 0x00000004389c0000-0x0000000438b5ffff]
> [    0.000000]   node   0: [mem 0x0000000438b60000-0x000000043be3ffff]
> [    0.000000]   node   0: [mem 0x000000043be40000-0x000000043becffff]
> [    0.000000]   node   0: [mem 0x000000043bed0000-0x000000043bedffff]
> [    0.000000]   node   0: [mem 0x000000043bee0000-0x000000043bffffff]
> [    0.000000]   node   0: [mem 0x000000043c000000-0x000000043fffffff]
>
> This is a pity really, because I don't see a fundamental reason for those
> tiny holes all over the place.
>

There is a config option in the firmware build that allows these
regions to be preallocated using larger windows, which greatly reduces
the fragmentation.
> I know that EFI/ACPI mandates "IO style" memory access for those regions,
> but I fail to get why...
>

Not sure what you mean by 'IO style memory access'.



> > > I beleive it would be best to route these via mmotm tree.
> > >
> > > v4:
> > > * rebase on v5.13-rc1
> > >
> > > v3: Link: https://lore.kernel.org/lkml/20210422061902.21614-1-rppt@kernel.org
> > > * Fix minor issues found by Anshuman
> > > * Freshen up the declaration of pfn_valid() to make it consistent with
> > >   pfn_is_map_memory()
> > > * Add more Acked-by and Reviewed-by tags, thanks Anshuman and David
> > >
> > > v2: Link: https://lore.kernel.org/lkml/20210421065108.1987-1-rppt@kernel.org
> > > * Add check for PFN overflow in pfn_is_map_memory()
> > > * Add Acked-by and Reviewed-by tags, thanks David.
> > >
> > > v1: Link: https://lore.kernel.org/lkml/20210420090925.7457-1-rppt@kernel.org
> > > * Add comment about the semantics of pfn_valid() as Anshuman suggested
> > > * Extend comments about MEMBLOCK_NOMAP, per Anshuman
> > > * Use pfn_is_map_memory() name for the exported wrapper for
> > >   memblock_is_map_memory(). It is still local to arch/arm64 in the end
> > >   because of header dependency issues.
> > >
> > > rfc: Link: https://lore.kernel.org/lkml/20210407172607.8812-1-rppt@kernel.org
> > >
> > > Mike Rapoport (4):
> > >   include/linux/mmzone.h: add documentation for pfn_valid()
> > >   memblock: update initialization of reserved pages
> > >   arm64: decouple check whether pfn is in linear map from pfn_valid()
> > >   arm64: drop pfn_valid_within() and simplify pfn_valid()
> > >
> > >  arch/arm64/Kconfig              |  3 ---
> > >  arch/arm64/include/asm/memory.h |  2 +-
> > >  arch/arm64/include/asm/page.h   |  3 ++-
> > >  arch/arm64/kvm/mmu.c            |  2 +-
> > >  arch/arm64/mm/init.c            | 14 +++++++++++++-
> > >  arch/arm64/mm/ioremap.c         |  4 ++--
> > >  arch/arm64/mm/mmu.c             |  2 +-
> > >  include/linux/memblock.h        |  4 +++-
> > >  include/linux/mmzone.h          | 11 +++++++++++
> > >  mm/memblock.c                   | 28 ++++++++++++++++++++++++++--
> > >  10 files changed, 60 insertions(+), 13 deletions(-)
> > >
> > >
> > > base-commit: 6efb943b8616ec53a5e444193dccf1af9ad627b5
> > > --
> > > 2.28.0
> > >
>
> --
> Sincerely yours,
> Mike.