Re: [PATCH v3 04/21] arm64: decouple early fixmap init from linear mapping

[Ard Biesheuvel]

On 11 January 2016 at 17:09, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> On Mon, Jan 11, 2016 at 02:18:57PM +0100, Ard Biesheuvel wrote:
>> Since the early fixmap page tables are populated using pages that are
>> part of the static footprint of the kernel, they are covered by the
>> initial kernel mapping, and we can refer to them without using __va/__pa
>> translations, which are tied to the linear mapping.
>>
>> Since the fixmap page tables are disjoint from the kernel mapping up
>> to the top level pgd entry, we can refer to bm_pte[] directly, and there
>> is no need to walk the page tables and perform __pa()/__va() translations
>> at each step.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
>> ---
>>  arch/arm64/mm/mmu.c | 32 ++++++--------------
>>  1 file changed, 9 insertions(+), 23 deletions(-)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 7711554a94f4..75b5f0dc3bdc 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -570,38 +570,24 @@ void vmemmap_free(unsigned long start, unsigned long end)
>>  #endif       /* CONFIG_SPARSEMEM_VMEMMAP */
>>
>>  static pte_t bm_pte[PTRS_PER_PTE] __page_aligned_bss;
>> -#if CONFIG_PGTABLE_LEVELS > 2
>>  static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss;
>> -#endif
>> -#if CONFIG_PGTABLE_LEVELS > 3
>>  static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss;
>> -#endif
>>
>>  static inline pud_t * fixmap_pud(unsigned long addr)
>>  {
>> -     pgd_t *pgd = pgd_offset_k(addr);
>> -
>> -     BUG_ON(pgd_none(*pgd) || pgd_bad(*pgd));
>> -
>> -     return pud_offset(pgd, addr);
>> +     return (CONFIG_PGTABLE_LEVELS > 3) ? &bm_pud[pud_index(addr)]
>> +                                        : (pud_t *)pgd_offset_k(addr);
>
> If we move patch 6 earlier, we could use pud_offset_kimg here, and avoid
> the cast, at the cost of passing the pgd into fixmap_pud.
>
> Similarly for fixmap_pmd.
>

Is that necessarily an improvement? I know it hides the cast, but I
think having an explicit pgd_t* to pud_t* cast that so obviously
applies to CONFIG_PGTABLE_LEVELS < 4 only is fine as well.

>>  }
>>
>> -static inline pmd_t * fixmap_pmd(unsigned long addr)
>> +static inline pte_t * fixmap_pmd(unsigned long addr)
>>  {
>> -     pud_t *pud = fixmap_pud(addr);
>> -
>> -     BUG_ON(pud_none(*pud) || pud_bad(*pud));
>> -
>> -     return pmd_offset(pud, addr);
>> +     return (CONFIG_PGTABLE_LEVELS > 2) ? &bm_pmd[pmd_index(addr)]
>> +                                        : (pmd_t *)pgd_offset_k(addr);
>>  }
>
> I assume the return type change was unintentional?
>

Yes. Thanks for spotting that.

> With STRICT_MM_TYPECHECKS:
>
> arch/arm64/mm/mmu.c: In function 'fixmap_pmd':
> arch/arm64/mm/mmu.c:604:9: warning: return from incompatible pointer type [-Wincompatible-pointer-types]
>   return (CONFIG_PGTABLE_LEVELS > 2) ? &bm_pmd[pmd_index(addr)]
>          ^
> arch/arm64/mm/mmu.c: In function 'early_fixmap_init':
> arch/arm64/mm/mmu.c:635:6: warning: assignment from incompatible pointer type [-Wincompatible-pointer-types]
>   pmd = fixmap_pmd(addr);
>       ^
> arch/arm64/mm/mmu.c:645:11: warning: comparison of distinct pointer types lacks a cast
>   if ((pmd != fixmap_pmd(fix_to_virt(FIX_BTMAP_BEGIN)))
>            ^
> arch/arm64/mm/mmu.c:646:14: warning: comparison of distinct pointer types lacks a cast
>        || pmd != fixmap_pmd(fix_to_virt(FIX_BTMAP_END))) {
>               ^
>
> Side note: is there any reason we can't/shouldn't make
> STRICT_MM_TYPECHECKS a common config option? Or simply have it on by
> default for arm64?
>

I wouldn't mind at all.

> Having built with and without typechecks I see that it doesn't bloat the
> kernel Image size, though the binary isn't quite identical:
>
> [mark@leverpostej:~/src/linux]% ls -al *.*checks
> -rwxrwxr-x 1 mark mark   9288192 Jan 11 15:40 Image.checks
> -rwxrwxr-x 1 mark mark   9288192 Jan 11 15:36 Image.nochecks
> -rwxrwxr-x 1 mark mark 106782024 Jan 11 15:40 vmlinux.checks
> -rwxrwxr-x 1 mark mark 106688928 Jan 11 15:35 vmlinux.nochecks
>
> Things didn't quite line up between the two images, though I'm not sure
> what the underlying difference was.
>
> Thanks,
> Mark.