[RFC/RFT,0/3] arm64: drop pfn_valid_within() and simplify pfn_valid()

Message

Mike Rapoport April 7, 2021, 5:26 p.m. UTC

From: Mike Rapoport <[e-mail]>

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 only boot tested on qemu-system-aarch64 so I'd really
appreciate memory stress tests on real hardware.

If this actually works we'll be one step closer to drop custom pfn_valid()
on arm64 altogether.

Mike Rapoport (3):
  memblock: update initialization of reserved pages
  arm64: decouple check whether pfn is normal memory 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   |  1 +
 arch/arm64/kvm/mmu.c            |  2 +-
 arch/arm64/mm/init.c            | 10 ++++++++--
 arch/arm64/mm/ioremap.c         |  4 ++--
 arch/arm64/mm/mmu.c             |  2 +-
 mm/memblock.c                   | 23 +++++++++++++++++++++--
 8 files changed, 35 insertions(+), 12 deletions(-)


base-commit: e49d033bddf5b565044e2abe4241353959bc9120

Comments

Anshuman Khandual April 8, 2021, 5:19 a.m. UTC | #1

Adding James here.

+ James Morse <[e-mail]>

On 4/7/21 10:56 PM, Mike Rapoport wrote:
> From: Mike Rapoport <[e-mail]>
> 
> Hi,
> 
> These patches aim to remove CONFIG_HOLES_IN_ZONE and essentially hardwire
> pfn_valid_within() to 1. 

That would be really great for arm64 platform as it will save CPU cycles on
many generic MM paths, given that our pfn_valid() has been expensive.

> 
> The idea is to mark NOMAP pages as reserved in the memory map and restore

Though I am not really sure, would that possibly be problematic for UEFI/EFI
use cases as it might have just treated them as normal struct pages till now.

> the intended semantics of pfn_valid() to designate availability of struct
> page for a pfn.

Right, that would be better as the current semantics is not ideal.

> 
> 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 only boot tested on qemu-system-aarch64 so I'd really
> appreciate memory stress tests on real hardware.

Did some preliminary memory stress tests on a guest with portions of memory
marked as MEMBLOCK_NOMAP and did not find any obvious problem. But this might
require some testing on real UEFI environment with firmware using MEMBLOCK_NOMAP
memory to make sure that changing these struct pages to PageReserved() is safe.


> 
> If this actually works we'll be one step closer to drop custom pfn_valid()
> on arm64 altogether.

Right, planning to rework and respin the RFC originally sent last month.

https://patchwork.kernel.org/project/linux-mm/patch/[e-mail]/

Mike Rapoport April 8, 2021, 6:27 a.m. UTC | #2

On Thu, Apr 08, 2021 at 10:49:02AM +0530, Anshuman Khandual wrote:
> Adding James here.
> 
> + James Morse <[e-mail]>
> 
> On 4/7/21 10:56 PM, Mike Rapoport wrote:
> > From: Mike Rapoport <[e-mail]>
> > 
> > Hi,
> > 
> > These patches aim to remove CONFIG_HOLES_IN_ZONE and essentially hardwire
> > pfn_valid_within() to 1. 
> 
> That would be really great for arm64 platform as it will save CPU cycles on
> many generic MM paths, given that our pfn_valid() has been expensive.
> 
> > 
> > The idea is to mark NOMAP pages as reserved in the memory map and restore
> 
> Though I am not really sure, would that possibly be problematic for UEFI/EFI
> use cases as it might have just treated them as normal struct pages till now.

I don't think there should be a problem because now the struct pages for
UEFI/ACPI never got to be used by the core mm. They were (rightfully)
skipped by memblock_free_all() from one side and pfn_valid() and
pfn_valid_within() return false for them in various pfn walkers from the
other side.
 
> > the intended semantics of pfn_valid() to designate availability of struct
> > page for a pfn.
> 
> Right, that would be better as the current semantics is not ideal.
> 
> > 
> > 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 only boot tested on qemu-system-aarch64 so I'd really
> > appreciate memory stress tests on real hardware.
> 
> Did some preliminary memory stress tests on a guest with portions of memory
> marked as MEMBLOCK_NOMAP and did not find any obvious problem. But this might
> require some testing on real UEFI environment with firmware using MEMBLOCK_NOMAP
> memory to make sure that changing these struct pages to PageReserved() is safe.

I surely have no access for such machines :)
 
> > If this actually works we'll be one step closer to drop custom pfn_valid()
> > on arm64 altogether.
> 
> Right, planning to rework and respin the RFC originally sent last month.
> 
> https://patchwork.kernel.org/project/linux-mm/patch/[e-mail]/