Re: [PATCH V3] powerpc/mm: Fix Multi hit ERAT cause by recent THP update

[Aneesh Kumar K.V]

Balbir Singh <bsingharora@xxxxxxxxx> writes:

>> Now we can't depend for mm_cpumask, a parallel find_linux_pte_hugepte
>> can happen outside that. Now i had a variant for kick_all_cpus_sync that
>> ignored idle cpus. But then that needs more verification.
>> 
>> http://article.gmane.org/gmane.linux.ports.ppc.embedded/81105
> Can be racy as a CPU moves from non-idle to idle
>
> In
>
>> +ÂÂÂÂÂpmd_hugepage_update(vma->vm_mm, address, pmdp, ~0UL, 0);
>> +ÂÂÂÂÂ/*
>> +ÂÂÂÂÂÂ* This ensures that generic code that rely on IRQ disabling
>> +ÂÂÂÂÂÂ* to prevent a parallel THP split work as expected.
>> +ÂÂÂÂÂÂ*/
>> +ÂÂÂÂÂkick_all_cpus_sync();
>
> pmdp_invalidate()->pmd_hugepage_update() can still run in parallel withÂ
> find_linux_pte_or_hugepte() and race.. Am I missing something?
>

Yes. But then we make sure that the pte_t returned by
find_linux_pte_or_hugepte doesn't change to a regular pmd entry by using
that kick. Now callers of find_lnux_pte_or_hugepte will check for
_PAGE_PRESENT. So if it called before
pmd_hugepage_update(_PAGE_PRESENT), we wait for the caller to finish the
usage (via kick()). Or they bail out after finding _PAGE_PRESENT cleared.

-aneesh