Hi Toshi, Andrew,
this patch(-set) is broken in several ways, please see below.
On Wed, Mar 14, 2018 at 12:01:55PM -0600, Toshi Kani wrote:
> Implement pud_free_pmd_page() and pmd_free_pte_page() on x86, which
> clear a given pud/pmd entry and free up lower level page table(s).
> Address range associated with the pud/pmd entry must have been purged
> by INVLPG.
An INVLPG before actually unmapping the page is useless, as other cores
or even speculative instruction execution can bring the TLB entry back
before the code actually unmaps the page.
> int pud_free_pmd_page(pud_t *pud)
> {
> - return pud_none(*pud);
> + pmd_t *pmd;
> + int i;
> +
> + if (pud_none(*pud))
> + return 1;
> +
> + pmd = (pmd_t *)pud_page_vaddr(*pud);
> +
> + for (i = 0; i < PTRS_PER_PMD; i++)
> + if (!pmd_free_pte_page(&pmd[i]))
> + return 0;
> +
> + pud_clear(pud);
TLB flush needed here, before the page is freed.
> + free_page((unsigned long)pmd);
> +
> + return 1;
> }
>
> /**
> @@ -724,6 +739,15 @@ int pud_free_pmd_page(pud_t *pud)
> */
> int pmd_free_pte_page(pmd_t *pmd)
> {
> - return pmd_none(*pmd);
> + pte_t *pte;
> +
> + if (pmd_none(*pmd))
> + return 1;
> +
> + pte = (pte_t *)pmd_page_vaddr(*pmd);
> + pmd_clear(pmd);
Same here, TLB flush needed.
Further this needs synchronization with other page-tables in the system
when the kernel PMDs are not shared between processes. In x86-32 with
PAE this causes a BUG_ON() being triggered at arch/x86/mm/fault.c:268
because the page-tables are not correctly synchronized.
> + free_page((unsigned long)pte);
> +
> + return 1;
> }
> #endif /* CONFIG_HAVE_ARCH_HUGE_VMAP */