Message-ID: <bc88d132-452a-42a1-9ee5-5407334d8aac@kernel.org>
Date: Wed, 26 Nov 2025 13:16:35 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>,
 Samuel Holland <samuel.holland@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Paul Walmsley <pjw@...nel.org>,
 linux-riscv@...ts.infradead.org, Andrew Morton <akpm@...ux-foundation.org>,
 linux-mm@...ck.org
Cc: devicetree@...r.kernel.org, Suren Baghdasaryan <surenb@...gle.com>,
 linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...nel.org>,
 Michal Hocko <mhocko@...e.com>, Conor Dooley <conor@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Alexandre Ghiti <alex@...ti.fr>,
 Emil Renner Berthing <kernel@...il.dk>, Rob Herring <robh+dt@...nel.org>,
 Vlastimil Babka <vbabka@...e.cz>, "Liam R . Howlett"
 <Liam.Howlett@...cle.com>, Julia Lawall <Julia.Lawall@...ia.fr>,
 Nicolas Palix <nicolas.palix@...g.fr>,
 Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v3 06/22] mm: Always use page table accessor functions

On 11/26/25 12:09, Ryan Roberts wrote:
> On 13/11/2025 01:45, 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)
>> )
> 
> There should absolutely never be any instances of core code directly setting an
> entry at any level. This *must* always go via the arch code helpers. Did you
> find any instances of this? If so, I would consider these bugs and suggest
> sending as a separate bugfix patch. Bad things could happen on arm64 because we
> may need to break a contiguous mapping, which would not happen if the value is
> set directly.
> 
>>
>> @@
>> 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)
>> )
> 
> For reading the *PTE*, conversion over to ptep_get() should have already been
> done (I did this a few years back when implementing support for arm64 contiguous
> mappings). If you find any cases where direct dereference or READ_ONCE() is
> being done in generic code for PTE, then that's a bug and should also be sent as
> a separate patch.
> 
> FYI, my experience was that Coccinelle didn't find everything when I was
> converting to ptep_get() - although it could have been that my Cochinelle skills
> were not up to scratch! I ended up using an additional method where I did a
> find/replace to convert "pte_t *" to "ptep_handle_t" and declared pte_handle_t
> as a void* which causes a compiler error on dereference. Then in a few key
> places I did a manual case from pte_handle_t to (pte_t *) and compiled allyesconfig.
> 
> I'm assuming the above Cocchinelle template was also used for pmd_t, pud_t,
> p4d_t and pgd_t?
> 
>>
>> 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/
> 
> Hi,
> 
> I've just come across this patch and wanted to mention that we could also
> benefit from this improved absraction for some features we are looking at for
> arm64. As you mention, Anshuman had a go but hit some roadblocks.
> 
> The main issue is that the compiler was unable to optimize away the READ_ONCE()s
> for the case where certain levels of the pgtable are folded. But it can optimize
> the plain C dereferences. There were complaints the the generated code for arm
> (32) and powerpc was significantly impacted due to having many more (redundant)
> loads.
> 

We do have mm_pmd_folded()/p4d_folded() etc, could that help to sort 
this out internally?

-- 
Cheers

David

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