Re: [PATCH v3 06/22] mm: Always use page table accessor functions

["David Hildenbrand (Red Hat)" <david@kernel.org>]

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@sifive.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.

-- 
Cheers

David