[PATCH] arm64:mm: free the useless initial page table

[Catalin Marinas]

On Tue, Nov 25, 2014 at 02:41:35PM +0000, zhichang.yuan wrote:
> On 2014年11月24日 22:32, Catalin Marinas wrote:
> > On Fri, Nov 21, 2014 at 08:27:40AM +0000, zhichang.yuan at linaro.org wrote:
> >> From: "zhichang.yuan" <zhichang.yuan at linaro.org>
> >>
> >> For 64K page system, after mapping a PMD section, the corresponding initial
> >> page table is not needed any more. That page can be freed.
> >>
> >> Signed-off-by: Zhichang Yuan <zhichang.yuan at linaro.org>
> >> ---
> >>  arch/arm64/mm/mmu.c |    5 ++++-
> >>  1 file changed, 4 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> >> index f4f8b50..12a336b 100644
> >> --- a/arch/arm64/mm/mmu.c
> >> +++ b/arch/arm64/mm/mmu.c
> >> @@ -191,8 +191,11 @@ static void __init alloc_init_pmd(pud_t *pud, unsigned long addr,
> >>  			 * Check for previous table entries created during
> >>  			 * boot (__create_page_tables) and flush them.
> >>  			 */
> >> -			if (!pmd_none(old_pmd))
> >> +			if (!pmd_none(old_pmd)) {
> >>  				flush_tlb_all();
> >> +				if (pmd_table(old_pmd))
> >> +					memblock_free(pte_pfn(pmd_pte(old_pmd)) << PAGE_SHIFT, PAGE_SIZE);
> >> +			}
> > For consistency with alloc_init_pud(), could you do:
> >
> > 	phys_addr_t table = __pa(pte_offset(&old_pmd, 0));
> > 	memblock_free(table, PAGE_SIZE);
>
> Just as Laura comments, the pte_offset will convert the PA to VA, and
> then convert  VA to PA. It seems to be verbose. That is why i use
> pte_pfn. Anyway, the consistency is better, i will revise it.
> 
> > Since you are at this, for alloc_init_pud() could you please move the
> > flush_tlb_all() before memblock_free()? Theoretical problem really but
> > it's nice for consistency.
>
> As for this issue, i have a small question.  Since the new PMD entry
> had been set by set_pmd, why not do the tlb flush as soon as we can?

So we need to flush the tlb shortly _after_ the new PMD or PUD has been
set to guarantee that a TLB load will see the new page table value. In
theory, we also need to flush it _before_ the old one is freed in case
the freed page is used by some other CPU (not a problem for boot code)
or some debug code poisons the freed page with a value that happens to
match a valid translation.

-- 
Catalin