Message-ID: <71123d7a-641b-41df-b959-88e6c2a3a441@kernel.org>
Date: Wed, 26 Nov 2025 13:35:17 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: 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, 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>, 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 13:27, Lorenzo Stoakes wrote:
> On Wed, Nov 26, 2025 at 01:19:00PM +0100, David Hildenbrand (Red Hat) wrote:
>> On 11/26/25 13:16, David Hildenbrand (Red Hat) wrote:
>>> 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?
>>>
>>
>> Just stumbled over the reply from Christope:
>>
>> https://lkml.kernel.org/r/0019d675-ce3d-4a5c-89ed-f126c45145c9@kernel.org
>>
>> And wonder if we could handle that somehow directly in the pgdp_get() etc.
> 
> I find that kind of gross to be honest. Isn't the whole point of folding that we
> don't have to think about it...

If we could adjust generic pgdp_get() and friends to not do a 
READ_ONCE() once folded we might not have to think about that in the 
callers.

Just an idea, though, not sure if that would fly the way I envision it.

-- 
Cheers

David

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