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
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[RFC] asm-generic/tlb: stub out pmd_free_tlb() if __PAGETABLE_PMD_FOLDED
This is inine with similar patches for nopud [1] and nop4d [2] cases.
However I'm not really sure I understand clearly how the nopmd code is
supposed to work (for a 2 tier paging system) - hence the RFC.
Consider free_pmd_range() simplified/annotated below
free_pmd_range
...
pmd = pmd_offset(pud, addr);
do {
next = pmd_addr_end(addr, end);
if (pmd_none_or_clear_bad(pmd)) => *pmd_bad()/pmd_clear_bad()
[a]*
continue;
free_pte_range(tlb, pmd, addr);
} while (pmd++, addr = next, addr != end);
...
*pmd_free_tlb(tlb, pmd, start); => [b]*
For ARC/nopmd case [a] is actually checking pgd and consequently
pmd_clear_bad() can't be stubbed out for PMD_FOLDED case. However it seems
case [b] can be stubbed out (hence this patch) along same lines as [1] and
[2]
| bloat-o-meter2 vmlinux-E-elide-p?d_clear_bad vmlinux-F-elide-pmd_free_tlb
| add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-112 (-112)
| function old new delta
| free_pgd_range 422 310-112
| Total: Before=4137002, After=4136890, chg -1.00%
[1] http://lists.infradead.org/pipermail/linux-snps-arc/2019-October/006266.html
[2] http://lists.infradead.org/pipermail/linux-snps-arc/2019-October/006265.html
Signed-off-by: Vineet Gupta
---
include/asm-generic/tlb.h | 4
1 file changed, 4 insertions(+)
diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index f3dad87f4ecc..a1edad7d4170 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -574,6 +574,7 @@ static inline void tlb_end_vma(struct mmu_gather *tlb,
struct vm_area_struct *vm
} while (0)
#endif
+#ifndef __PAGETABLE_PMD_FOLDED
#ifndef pmd_free_tlb
#define pmd_free_tlb(tlb, pmdp, address) \
do {\
@@ -583,6 +584,9 @@ static inline void tlb_end_vma(struct mmu_gather *tlb,
struct vm_area_struct *vm
__pmd_free_tlb(tlb, pmdp, address); \
} while (0)
#endif
+#else
+#define pmd_free_tlb(tlb, pmdp, address)do { } while (0)
+#endif
#ifndef __PAGETABLE_PUD_FOLDED
#ifndef pud_free_tlb
--
2.20.1
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