From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Vineet Gupta <vineetg76@gmail.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Vineet Gupta <Vineet.Gupta1@synopsys.com>,
linux-arch@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
Peter Zijlstra <peterz@infradead.org>,
"Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
linux-kernel@vger.kernel.org, Nick Piggin <npiggin@gmail.com>,
linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
linux-snps-arc@lists.infradead.org, Will Deacon <will@kernel.org>
Subject: Re: [PATCH 0/3] eldie generated code for folded p4d/pud
Date: Fri, 11 Oct 2019 15:19:51 +0300 [thread overview]
Message-ID: <20191011121951.nxna6hruuskvdxod@box> (raw)
In-Reply-To: <8ba067a6-8b6a-2414-0f04-b251cd6bb47c@gmail.com>
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 +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.
>
> 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.mfjgkf47kdp2by4e@black.fi.intel.com/
--
Kirill A. Shutemov
next prev parent reply other threads:[~2019-10-11 12:20 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-09 22:26 [PATCH 0/3] eldie generated code for folded p4d/pud Vineet Gupta
2019-10-09 22:26 ` [PATCH 1/3] asm-generic/tlb: stub out pud_free_tlb() if __PAGETABLE_PUD_FOLDED Vineet Gupta
2019-10-09 22:26 ` [PATCH 2/3] asm-generic/tlb: stub out p4d_free_tlb() if __PAGETABLE_P4D_FOLDED Vineet Gupta
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 Gupta
2019-10-10 7:29 ` [PATCH 0/3] eldie generated code for folded p4d/pud Peter Zijlstra
2019-10-10 8:56 ` Kirill A. Shutemov
2019-10-10 20:05 ` Vineet Gupta
2019-10-11 12:19 ` Kirill A. Shutemov [this message]
2019-10-11 22:38 ` [RFC] asm-generic/tlb: stub out pmd_free_tlb() if __PAGETABLE_PMD_FOLDED Vineet Gupta
2019-10-14 17:41 ` Linus Torvalds
2019-10-14 18:02 ` Vineet Gupta
2019-10-14 18:25 ` Linus Torvalds
2019-10-14 19:08 ` Vineet Gupta
2019-10-14 20:38 ` Linus Torvalds
2019-10-14 21:48 ` Matthew Wilcox
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