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From: (Vineet Gupta)
Subject: [PATCH 0/3] eldie generated code for folded p4d/pud
Date: Thu, 10 Oct 2019 13:05:56 -0700
Message-ID: <> (raw)
In-Reply-To: <>

Hi Kirill,

On 10/10/19 1:56 AM, Kirill A. Shutemov wrote:
> On Wed, Oct 09, 2019@10:26:55PM +0000, 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.


So given we are folding pmd too, it seemed we could do the following as well.

 void pmd_clear_bad(pmd_t *);
+#define pmd_clear_bad(pmd)        do { } while (0)

 void pmd_clear_bad(pmd_t *pmd)

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.


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Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-09 22:26 Vineet.Gupta1
2019-10-09 22:26 ` [PATCH 1/3] asm-generic/tlb: stub out pud_free_tlb() if __PAGETABLE_PUD_FOLDED Vineet.Gupta1
2019-10-09 22:26 ` [PATCH 2/3] asm-generic/tlb: stub out p4d_free_tlb() if __PAGETABLE_P4D_FOLDED Vineet.Gupta1
2019-10-09 22:26 ` [PATCH 3/3] asm-generic/mm: stub out p{4, d}d_clear_bad() if __PAGETABLE_P{4, u}D_FOLDED Vineet.Gupta1
2019-10-10  7:29 ` [PATCH 0/3] eldie generated code for folded p4d/pud peterz
2019-10-10  8:56 ` kirill.shutemov
2019-10-10 20:05   ` vineetg76 [this message]
2019-10-11 12:19     ` kirill
2019-10-11 22:38       ` [RFC] asm-generic/tlb: stub out pmd_free_tlb() if __PAGETABLE_PMD_FOLDED Vineet.Gupta1

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