linux-kernel - Re: [PATCH v2 1/5] ARC: mm: remove __ARCH_USE_5LEVEL

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Date:   Tue, 15 Oct 2019 15:06:41 -0700
From:   Vineet Gupta <vineetg76@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-arch <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 Mailing List <linux-kernel@...r.kernel.org>,
        Nick Piggin <npiggin@...il.com>, Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-snps-arc@...ts.infradead.org, Will Deacon <will@...nel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v2 1/5] ARC: mm: remove __ARCH_USE_5LEVEL_HACK

On 10/15/19 2:48 PM, Linus Torvalds wrote:
> On Tue, Oct 15, 2019 at 12:19 PM Vineet Gupta
> <Vineet.Gupta1@...opsys.com> wrote:
>> This is a non-functional change anyways since ARC has software page walker
>> with 2 lookup levels (pgd -> pte)
>
> Could we encourage other architectures to do the same, and get rid of
> all uses of __ARCH_USE_5LEVEL_HACK?

IMHO this should have been done at the onset. The actual changes don't seem that
difficult, just need to add the missing p4d calls although I can sympathize with
hassles of coordinating/building/testing/yadi yada cross arch.

OTOH, the [45]LEVEL_HACK seem like a nice way to "fold" the levels: the
skipped/folded level vanishes completely. Among others it does things like

     #define p4d_t	pgd_t	

On ARC 2-level code:

   free_pte_range
	pmd_pgtable(*pmd);

	    ((((((*pmd).pud).pgd))) & PAGE_MASK));	     <-- 5LEVEL_HACK
			vs.
	    ((((((((*pmd).pud).p4d).pgd)))) & PAGE_MASK ));  <-- w/o 5LEVEL_HACK
	
	pmd_clear(pmd);

	    *(pmd)).pud).pgd)))) = 0
			vs.
	    *(pmd)).pud).p4d).pgd)))) = 0


So we may not be able to fix all he historical misgivigs, but this might alleviate
the pain a bit. I'll try to dabble a bit.

Thx for taking a look and te ACKs.
-Vineet