On Fri, Apr 24, 2020 at 09:33:40PM +0800, Li Xinhai wrote:
> >
> >> , so when sz == PMD_SIZE, pmd_offset() only called with a valid PUD
> >> entry which point to PMD page table.
> >
> >But what prevents pud_huge?
> >
> if sz == PUD_SIZE, the 'return (pte_t*)pud' alrady end the function, which cover
> pud_huge() and pud_none(), because we the mapping is for PUD_SIZE huge page.
>
> So, there is no possibility for pmd_offset() been called with invalid pud entry.
> Below is the code I used for test which has BUG_ON, that should give more
> clear idea about the semantics of code path:
>
> ...
> pud = pud_offset(p4d, addr);
> if (sz == PUD_SIZE) {
> /* must be pud_huge or pud_none */
> BUG_ON(!pud_huge(*pud) && !pud_none(*pud));
> return (pte_t *)pud; // note that return valid pointer for pud_none() case,
> // instead of NULL, that is same semantics as existing code.
> }
> if (!pud_present(*pud))
> return NULL; // note that only return NULL in case pud not present,
> // same sematics as existing code.
> /* must have a valid entry and size to go further */
> BUG_ON(sz != PMD_SIZE);
>
> pmd = pmd_offset(pud, addr);
> /* must be pmd_huge or pmd_none */
> BUG_ON(!pmd_huge(*pmd) && !pmd_none(*pmd));
But why is !pmd_huge() ? The prior code returned null here, is that
dead code? Your commit message should explain all of this..
Jason
