Re: [PATCH -v2 1/2] mm: add spurious fault fixing support for huge pmd

["Huang, Ying" <ying.huang@linux.alibaba.com>]

Lorenzo Stoakes <lorenzo.stoakes@oracle.com> writes:

> On Thu, Oct 16, 2025 at 10:22:57AM +0800, Huang, Ying wrote:
>> Lorenzo Stoakes <lorenzo.stoakes@oracle.com> writes:
>>
>> > On Wed, Oct 15, 2025 at 04:43:14PM +0800, Huang, Ying wrote:
>> > OK this is great, let's put it all in the kdoc for the new shared spurious
>> > faulting function! :) and additionally add it to the commit message.
>>
>> Sure.  Will do it in the next version.
>
> Thanks!
>
>>
>> [snip]
>>
>> >> >>
>> >> >> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>> >> >> index 32e8457ad535..341622ec80e4 100644
>> >> >> --- a/include/linux/pgtable.h
>> >> >> +++ b/include/linux/pgtable.h
>> >> >> @@ -1232,6 +1232,10 @@ static inline void arch_swap_restore(swp_entry_t entry, struct folio *folio)
>> >> >>  #define flush_tlb_fix_spurious_fault(vma, address, ptep) flush_tlb_page(vma, address)
>> >> >>  #endif
>> >> >>
>> >> >> +#ifndef flush_tlb_fix_spurious_fault_pmd
>> >> >> +#define flush_tlb_fix_spurious_fault_pmd(vma, address, ptep) do { } while (0)
>> >> >> +#endif
>> >> >
>> >> > flush_tlb_fix_spurious_fault(), when the arch doesn't declare it, defaults to
>> >> > flush_tlb_page() - why do we just do nothing in this case here?
>> >>
>> >> Because all architectures do nothing for the spurious PMD page fault
>> >> fixing until the [2/2] of this series.  Where, we make it necessary to
>> >> flush the local TLB for spurious PMD page fault fixing on arm64
>> >> architecture.
>> >>
>> >> If we follow the design of flush_tlb_fix_spurious_fault(), we need to
>> >> change all architecture implementation to do nothing in this patch to
>> >> keep the current behavior.  I don't think that it's a good idea.  Do
>> >> you agree?
>> >
>> > Yeah probably we should keep the same behaviour as before, which is
>> > obviously, prior to this series, we did nothing.
>> >
>> > I guess in the PTE case we _always_ want to flush the TLB, whereas in the
>> > PMD case we otherwise don't have any need to at the point at which the
>> > spurious flush is performed.
>> >
>> > But from your explanation above re: the stale TLB entry this _only_ needs
>> > to be done for architectures which might encounter this problem rather than
>> > needing a TLB flush in general.
>> >
>> > Given we're generalising the code and one case always flushes the TLB and
>> > the other doesn't maybe it's worth putting a comment in the generalised
>> > function mentioning this?
>>
>> I'm not sure whether it's a good idea to document architecture behaviors
>> in the general code.  The behavior may be changed architecture by
>> architecture in the future.
>
> Right, but we are unconditionaly doing a TLB flush in the PTE case but not PMD
> so let's document that to be clear :)

Sure.  Will do this.

>>
>> [snip]

---
Best Regards,
Huang, Ying