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

[Mike Rapoport]

On Thu, Apr 08, 2021 at 10:49:02AM +0530, Anshuman Khandual wrote:
> Adding James here.
> 
> + James Morse <james.morse@xxxxxxx>
> 
> On 4/7/21 10:56 PM, Mike Rapoport wrote:
> > From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > 
> > 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/1615174073-10520-1-git-send-email-anshuman.khandual@xxxxxxx/

-- 
Sincerely yours,
Mike.