linux-kernel - Re: [PATCH 00/10] Account page tables at all levels

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Message-ID: <80cf6cae-11f6-4db2-816b-b1dcca3cee3e@arm.com>
Date: Fri, 3 Jan 2025 10:28:32 +0100
From: Kevin Brodsky <kevin.brodsky@....com>
To: Dave Hansen <dave.hansen@...el.com>, linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Catalin Marinas <catalin.marinas@....com>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Linus Walleij <linus.walleij@...aro.org>, Andy Lutomirski <luto@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, "Mike Rapoport (IBM)"
 <rppt@...nel.org>, Ryan Roberts <ryan.roberts@....com>,
 Thomas Gleixner <tglx@...utronix.de>, Will Deacon <will@...nel.org>,
 Matthew Wilcox <willy@...radead.org>, linux-alpha@...r.kernel.org,
 linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-csky@...r.kernel.org, linux-hexagon@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
 linux-mips@...r.kernel.org, linux-openrisc@...r.kernel.org,
 linux-parisc@...r.kernel.org, linux-riscv@...ts.infradead.org,
 linux-s390@...r.kernel.org, linux-snps-arc@...ts.infradead.org,
 linux-um@...ts.infradead.org, loongarch@...ts.linux.dev, x86@...nel.org,
 Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 00/10] Account page tables at all levels

On 20/12/2024 20:31, Dave Hansen wrote:
> On 12/20/24 02:58, Kevin Brodsky wrote:
>>> One super tiny nit is that the PAE pgd _can_ be allocated using
>>> __get_free_pages(). It was originally there for Xen, but I think it's
>>> being used for PTI only at this point and the comments are wrong-ish.
>>>
>>> I kinda think we should just get rid of the 32-bit kmem_cache entirely.
>> That would certainly simplify things on the x86 side! I'm not at all
>> familiar with that code though, would you be happy with providing a
>> patch? I could add it to this series if that's convenient.
> I hacked this together yesterday:
>
>> https://git.kernel.org/pub/scm/linux/kernel/git/daveh/devel.git/log/?h=simplify-pae-20241220
> It definitely needs some more work. I'm particularly still puzzling
> about why SHARED_KERNEL_PMD is used both as a trigger for 32b vs.
> PAGE_SIZE PAE pgd allocations _and_ for the actual PMD sharing.
>
> Xen definitely needed the whole page behavior but I'm not sure why PTI did.
>
> Either way, that series should make the PAE PGDs a _bit_ less weird at
> the cost of an extra ~2 pages per process for folks who are running
> 32-bit PAE kernels with PTI disabled.
>
> But I think the diffstat is worth it:
>
>  5 files changed, 16 insertions(+), 96 deletions(-)

That does look like a nice simplification! After the first patch, with
my series, we could get rid of _pgd_alloc() and _pgd_free() in
arch/x86/mm/pgtable.c and just call __pgd_alloc() and __pgd_free() directly.

Considering that these changes are not trivial and may need more work,
should I let you post those patches as a separate series? If it gets
merged soon, I'll adapt my series, otherwise I can post a follow-up
patch later if needed.

- Kevin