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Message-ID: <adfe981a-ec9a-4051-a26f-91b691230161@arm.com>
Date: Fri, 30 May 2025 11:03:34 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org, david@...hat.com,
catalin.marinas@....com, will@...nel.org
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz,
rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, suzuki.poulose@....com, steven.price@....com,
gshan@...hat.com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 0/3] Enable huge-vmalloc permission change
On 30/05/2025 10:04, Dev Jain wrote:
> This series paves the path to enable huge mappings in vmalloc space by
> default on arm64. > For this we must ensure that we can handle any permission
> games on vmalloc space.
And the linear map :)
> Currently, __change_memory_common() uses
> apply_to_page_range() which does not support changing permissions for
> leaf mappings.
nit: A "leaf mapping" is the lowest level entry in the page tables for a given
address - i.e. it maps an address to some actual memory rather than to another
pgtable. It includes what the Arm ARM calls "page mappings" (PTE level) and
"block mappings" (PMD/PUD/.. level). apply_to_page_range() does support page
mappings, so saying it doesn't support leaf mappings is incorrect. It doesn't
support block mappings.
> We attempt to move away from this by using walk_page_range_novma(),
> similar to what riscv does right now; however, it is the responsibility
> of the caller to ensure that we do not pass a range, or split the range
> covering a partial leaf mapping.
>
> This series is tied with Yang Shi's attempt [1] at using huge mappings
> in the linear mapping in case the system supports BBML2, in which case
> we will be able to split the linear mapping if needed without break-before-make.
> Thus, Yang's series, IIUC, will be one such user of my series; suppose we
> are changing permissions on a range of the linear map backed by PMD-hugepages,
> then the sequence of operations should look like the following:
>
> split_range(start, (start + HPAGE_PMD_SIZE) & ~HPAGE_PMD_MASK);
> split_range(end & ~HPAGE_PMD_MASK, end);
I don't understand what the HPAGE_PMD_MASK twiddling is doing? That's not right.
It's going to give you the offset within the 2M region. You just want:
split_range(start)
split_range(end)
right?
> __change_memory_common(start, end);
>
> However, this series can be used independently of Yang's; since currently
> permission games are being played only on pte mappings (due to apply_to_page_range
> not supporting otherwise), this series provides the mechanism for enabling
> huge mappings for various kernel mappings like linear map and vmalloc.
In other words, you are saying that this series is a prerequisite for Yang's
series (and both are prerequisites for huge vmalloc by default). Your series
adds a new capability that Yang's series will rely on (the ability to change
permissions on block mappings).
Thanks,
Ryan
>
> [1] https://lore.kernel.org/all/20250304222018.615808-1-yang@os.amperecomputing.com/
>
> Dev Jain (3):
> mm: Allow pagewalk without locks
> arm64: pageattr: Use walk_page_range_novma() to change memory
> permissions
> mm/pagewalk: Add pre/post_pte_table callback for lazy MMU on arm64
>
> arch/arm64/mm/pageattr.c | 81 +++++++++++++++++++++++++++++++++++++---
> include/linux/pagewalk.h | 4 ++
> mm/pagewalk.c | 18 +++++++--
> 3 files changed, 94 insertions(+), 9 deletions(-)
>
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