linux-kernel - Re: [PATCH v2 04/18] mm: Always use page table accessor functions

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Message-ID: <c6371fa1-af9c-47b0-988f-aae3d7fd58e7@arm.com>
Date: Thu, 9 Oct 2025 07:40:10 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Samuel Holland <samuel.holland@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Paul Walmsley <pjw@...nel.org>,
 linux-riscv@...ts.infradead.org
Cc: devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, Conor Dooley <conor@...nel.org>,
 Alexandre Ghiti <alex@...ti.fr>, Emil Renner Berthing <kernel@...il.dk>,
 Andrew Morton <akpm@...ux-foundation.org>, Rob Herring <robh+dt@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>
Subject: Re: [PATCH v2 04/18] mm: Always use page table accessor functions



On 09/10/25 7:27 AM, Samuel Holland wrote:
> Some platforms need to fix up the values when reading or writing page
> tables. Because of this, the accessors must always be used; it is not
> valid to simply dereference a pXX_t pointer.
> 
> Fix all of the instances of this pattern in generic code, mostly by
> applying the below coccinelle semantic patch, repeated for each page
> table level. Some additional fixes were applied manually, mostly to
> macros where type information is unavailable.
> 
> In a few places, a `pte_t *` or `pmd_t *` is actually a pointer to a PTE
> or PMDE value stored on the stack, not a pointer to a page table. In
> those cases, it is not appropriate to use the accessors, because the
> value is not globally visible, and any transformation from pXXp_get()
> has already been applied. Those places are marked by naming the pointer
> `ptentp` or `pmdvalp`, as opposed to `ptep` or `pmdp`.
> 
> @@
> pte_t *P;
> expression E;
> expression I;
> @@
> - P[I] = E
> + set_pte(P + I, E)
> 
> @@
> pte_t *P;
> expression E;
> @@
> (
> - WRITE_ONCE(*P, E)
> + set_pte(P, E)
> |
> - *P = E
> + set_pte(P, E)
> )
> 
> @@
> pte_t *P;
> expression I;
> @@
> (
>   &P[I]
> |
> - READ_ONCE(P[I])
> + ptep_get(P + I)
> |
> - P[I]
> + ptep_get(P + I)
> )
> 
> @@
> pte_t *P;
> @@
> (
> - READ_ONCE(*P)
> + ptep_get(P)
> |
> - *P
> + ptep_get(P)
> )
> 
> Additionally, the following semantic patch was used to convert PMD and
> PUD references inside struct vm_fault:
> 
> @@
> struct vm_fault vmf;
> @@
> (
> - *vmf.pmd
> + pmdp_get(vmf.pmd)
> |
> - *vmf.pud
> + pudp_get(vmf.pud)
> )
> 
> @@
> struct vm_fault *vmf;
> @@
> (
> - *vmf->pmd
> + pmdp_get(vmf->pmd)
> |
> - *vmf->pud
> + pudp_get(vmf->pud)
> )
> 
> Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
> ---
> This commit covers some of the same changes as an existing series from
> Anshuman Khandual[1]. Unlike that series, this commit is a purely
> mechanical conversion to demonstrate the RISC-V changes, so it does not
> insert local variables to avoid redundant calls to the accessors. A
> manual conversion like in that series could improve performance.
> 
> [1]: https://lore.kernel.org/linux-mm/20240917073117.1531207-1-anshuman.khandual@arm.com/

Portions of the changes proposed here are already in discussion
and fetched for testing via mm-new/everything.

https://lore.kernel.org/linux-mm/20251007063100.2396936-1-anshuman.khandual@arm.com/
https://lore.kernel.org/linux-mm/20251006055214.1845342-1-anshuman.khandual@arm.com/