Re: [PATCH v8 03/10] mm/vmalloc: use pte_set_huge()/pte_clear_huge() for PTE-level block mappings

["Christophe Leroy (CS GROUP)" <chleroy@kernel.org>]

Le 18/09/2026 à 08:14, Barry Song a écrit :
> On Fri, Sep 18, 2026 at 2:05 PM Christophe Leroy (CS GROUP)
> <chleroy@kernel.org> wrote:
>>
>>
>>
>> Le 17/09/2026 à 23:44, Barry Song a écrit :
>>> On Thu, Sep 17, 2026 at 10:41 PM Wen Jiang <jiangwenxiaomi@gmail.com> wrote:
> [...]
> 
>>>>>>
>>>>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>>>>>> index cdd68ed3ae1a9..349ced999f959 100644
>>>>>> --- a/include/linux/pgtable.h
>>>>>> +++ b/include/linux/pgtable.h
>>>>>> @@ -2134,6 +2134,35 @@ static inline int pmd_free_pte_page(pmd_t *pmd, unsigned long addr)
>>>>>>     }
>>>>>>     #endif      /* CONFIG_HAVE_ARCH_HUGE_VMAP */
>>>>>>
>>>>>> +/*
>>>>>> + * PTE-level block mappings for vmap.
>>>>>> + *
>>>>>> + * pte_set_huge() only has to be implemented by architectures whose
>>>>>> + * arch_vmap_pte_range_map_size() can return a size other than PAGE_SIZE.
>>>>>> + */
>>>>>> +#ifndef __HAVE_ARCH_PTE_SET_HUGE
>>>>>> +static inline void pte_set_huge(pte_t *ptep, unsigned long addr,
>>>>>> +                             phys_addr_t phys, pgprot_t prot,
>>>>>> +                             unsigned long size)
>>>>>> +{
>>>>>> +     WARN_ON_ONCE(1);
>>>>>
>>>>> BUILD_BUG_ON() would be better here.
>>>>>
>>>>> It should be possible because fallback arch_vmap_pte_range_map_size()
>>>>> will constant-fold PAGE_SIZE so pte_set_huge() will never be called.
>>>>>
>>>>
>>>> Agreed. These fallbacks exist only to keep the build working on
>>>> architectures with PTE-level block mappings and should never actually
>>>> be reached, so BUILD_BUG_ON() is right. I'll make that change in v9.
>>>>
>>>
>>> I am not quite sure. It won't be called at runtime because
>>> `vmap size`/`unmap size` return `PAGE_SIZE`, so the code won't
>>> reach this branch. But it will still be built.
>>
>> The fallbacks are defined as:
>>
>> #ifndef arch_vmap_pte_range_map_size
>> static inline unsigned long arch_vmap_pte_range_map_size(unsigned long
>> addr, unsigned long end,
>>                                                           u64 pfn, unsigned int max_page_shift)
>> {
>>          return PAGE_SIZE;
>> }
>> #endif
>>
>> #ifndef arch_vmap_pte_range_unmap_size
>> static inline unsigned long arch_vmap_pte_range_unmap_size(unsigned long
>> addr,
>>                                                             pte_t *ptep)
>> {
>>          return PAGE_SIZE;
>> }
>> #endif
>>
>> Therefore in:
>>
>>                   size = arch_vmap_pte_range_unmap_size(addr, pte);
>>                   if (size != PAGE_SIZE) {
>>
>> GCC knows 'size' is const and its value is PAGE_SIZE, so it won't emit
>> the branch at all.
>>
>> It is call constant folding, some explanation here:
>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FConstant_folding&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C869dac21f28d45988a3d08df154c225a%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639253088878897551%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=Dvtz1%2FAHvAbVYO%2BRaHTOizcD%2FclJ9FqTcWXKMhIFVyw%3D&reserved=0
>>
>>>
>>> So, would a `BUILD_BUG_ON()` trigger a build failure here?
>>
>> It shouldn't, if it does it is a compiled bug or this is because someone
>> has redefined arch_vmap_pte_range_map_size() and not pte_set_huge()
>> which we'd better know at build time rather than at runtime.
> 
> Thanks, Christophe. I was also thinking about compiler optimization. I
> was just a bit worried that we're touching the common MM code, which
> affects almost all architectures, so I'm not quite sure whether this is
> supported by all GCC versions used by those architectures.
> 
> If it is supported by all of them, I agree that `BUILD_BUG_ON()` is a
> perfect approach.

AFAIU this is the assumption made by the kernel, see 
https://docs.kernel.org/process/coding-style.html#conditional-compilation

This is the same compiler, I see no reason why ability to constant-fold 
would be dependant on architecture.

Christophe