linux-kernel - Re: [PATCH 0/3] eldie generated code for folded p4d/pud

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Date:   Thu, 10 Oct 2019 13:05:56 -0700
From:   Vineet Gupta <vineetg76@...il.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:     linux-arch@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, Nick Piggin <npiggin@...il.com>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        linux-snps-arc@...ts.infradead.org, Will Deacon <will@...nel.org>
Subject: 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 +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.

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