Re: [PATCH 0/3] eldie generated code for folded p4d/pud
On Thu, Oct 10, 2019 at 01:05:56PM -0700, Vineet Gupta wrote:
>
> Hi Kirill,
>
> On 10/10/19 1:56 AM, Kirill A. Shutemov wrote:
> > On Wed, Oct 09, 2019 at 10:26:55PM +, Vineet Gupta wrote:
> >>
> >> This series elides extraneous generate code for folded p4d/pud.
> >> This came up when trying to remove __ARCH_USE_5LEVEL_HACK from ARC port.
> >> The code saving are not a while lot, but still worthwhile IMHO.
> >
> > Agreed.
>
> Thx.
>
> So given we are folding pmd too, it seemed we could do the following as well.
>
> +#ifndef __PAGETABLE_PMD_FOLDED
> void pmd_clear_bad(pmd_t *);
> +#else
> +#define pmd_clear_bad(pmd)do { } while (0)
> +#endif
>
> +#ifndef __PAGETABLE_PMD_FOLDED
> void pmd_clear_bad(pmd_t *pmd)
> {
> pmd_ERROR(*pmd);
> pmd_clear(pmd);
> }
> +#endif
>
> I stared at generated code and it seems a bit wrong.
> free_pgd_range() -> pgd_none_or_clear_bad() is no longer checking for
> unmapped pgd
> entries as pgd_none/pgd_bad are all stubs returning 0.
>
> This whole pmd folding is a bit confusing considering I only revisit it every
> few
> years :-) Abstraction wise, __PAGETABLE_PMD_FOLDED only has pgd, pte but even
> in
> this regime bunch of pmd macros are still valid
>
> pmd_set(pmdp, ptep) {
> *pmdp.pud.p4d.pgd = (unsigned long)ptep
> }
>
> Is there a better way to make a mental model of this code folding.
I don't have any. PMD folding predates me and have never looked at it
closely. Quick look brings more confusion than clarity. :P
> In an ideal world pmd folded would have meant pmd_* routines just vanish -
> poof.
> So in that sense I like your implementation under #[45]LEVEL_HACK where the
> level
> simply vanishes by code like #define p4d_t pgd_t. Perhaps there is lot of
> historic
> baggage, proliferated into arch code so hard to untangle.
In ideal world all these pgd/p4d/pud/pmd/pte should die and we have
something more flexible to begin with.
I played with this before:
https://lore.kernel.org/lkml/20180424154355.mfjgkf47kdp2b...@black.fi.intel.com/
--
Kirill A. Shutemov
Re: [PATCH 0/3] eldie generated code for folded p4d/pud
Hi Kirill,
On 10/10/19 1:56 AM, Kirill A. Shutemov wrote:
> On Wed, Oct 09, 2019 at 10:26:55PM +, Vineet Gupta wrote:
>>
>> This series elides extraneous generate code for folded p4d/pud.
>> This came up when trying to remove __ARCH_USE_5LEVEL_HACK from ARC port.
>> The code saving are not a while lot, but still worthwhile IMHO.
>
> Agreed.
Thx.
So given we are folding pmd too, it seemed we could do the following as well.
+#ifndef __PAGETABLE_PMD_FOLDED
void pmd_clear_bad(pmd_t *);
+#else
+#define pmd_clear_bad(pmd)do { } while (0)
+#endif
+#ifndef __PAGETABLE_PMD_FOLDED
void pmd_clear_bad(pmd_t *pmd)
{
pmd_ERROR(*pmd);
pmd_clear(pmd);
}
+#endif
I stared at generated code and it seems a bit wrong.
free_pgd_range() -> pgd_none_or_clear_bad() is no longer checking for unmapped
pgd
entries as pgd_none/pgd_bad are all stubs returning 0.
This whole pmd folding is a bit confusing considering I only revisit it every
few
years :-) Abstraction wise, __PAGETABLE_PMD_FOLDED only has pgd, pte but even in
this regime bunch of pmd macros are still valid
pmd_set(pmdp, ptep) {
*pmdp.pud.p4d.pgd = (unsigned long)ptep
}
Is there a better way to make a mental model of this code folding.
In an ideal world pmd folded would have meant pmd_* routines just vanish - poof.
So in that sense I like your implementation under #[45]LEVEL_HACK where the
level
simply vanishes by code like #define p4d_t pgd_t. Perhaps there is lot of
historic
baggage, proliferated into arch code so hard to untangle.
Thx,
-Vineet
Re: [PATCH 0/3] eldie generated code for folded p4d/pud
On Wed, Oct 09, 2019 at 10:26:55PM +, Vineet Gupta wrote: > Hi, > > This series elides extraneous generate code for folded p4d/pud. > This came up when trying to remove __ARCH_USE_5LEVEL_HACK from ARC port. > The code saving are not a while lot, but still worthwhile IMHO. Agreed. Acked-by: Kirill A. Shutemov -- Kirill A. Shutemov
Re: [PATCH 0/3] eldie generated code for folded p4d/pud
On Wed, Oct 09, 2019 at 03:26:55PM -0700, Vineet Gupta wrote: > Hi, > > This series elides extraneous generate code for folded p4d/pud. > This came up when trying to remove __ARCH_USE_5LEVEL_HACK from ARC port. > The code saving are not a while lot, but still worthwhile IMHO. > > bloat-o-meter2 vmlinux-A-baseline vmlinux-E-elide-p?d_clear_bad > add/remove: 0/2 grow/shrink: 0/1 up/down: 0/-146 (-146) > function old new delta > p4d_clear_bad 2 - -2 > pud_clear_bad 20 - -20 > free_pgd_range 546 422-124 > Total: Before=4137148, After=4137002, chg -1.00% > Works for me, thanks!
[PATCH 0/3] eldie generated code for folded p4d/pud
Hi,
This series elides extraneous generate code for folded p4d/pud.
This came up when trying to remove __ARCH_USE_5LEVEL_HACK from ARC port.
The code saving are not a while lot, but still worthwhile IMHO.
bloat-o-meter2 vmlinux-A-baseline vmlinux-E-elide-p?d_clear_bad
add/remove: 0/2 grow/shrink: 0/1 up/down: 0/-146 (-146)
function old new delta
p4d_clear_bad 2 - -2
pud_clear_bad 20 - -20
free_pgd_range 546 422-124
Total: Before=4137148, After=4137002, chg -1.00%
Thx,
-Vineet
Vineet Gupta (3):
asm-generic/tlb: stub out pud_free_tlb() if __PAGETABLE_PUD_FOLDED ...
asm-generic/tlb: stub out p4d_free_tlb() if __PAGETABLE_P4D_FOLDED ...
asm-generic/mm: stub out p{4,d}d_clear_bad() if
__PAGETABLE_P{4,u}D_FOLDED
include/asm-generic/4level-fixup.h | 2 --
include/asm-generic/5level-fixup.h | 2 --
include/asm-generic/pgtable.h | 11 +++
include/asm-generic/tlb.h | 8 ++--
mm/pgtable-generic.c | 4
5 files changed, 21 insertions(+), 6 deletions(-)
--
2.20.1
