linux-kernel - Re: [PATCH v9 09/12] mm/kasan: kasan specific map populate function

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Date:   Mon, 9 Oct 2017 19:48:34 +0100
From:   Will Deacon <will.deacon@....com>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
Cc:     Mark Rutland <mark.rutland@....com>, catalin.marinas@....com,
        linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        x86@...nel.org, kasan-dev@...glegroups.com, borntraeger@...ibm.com,
        heiko.carstens@...ibm.com, davem@...emloft.net,
        willy@...radead.org, mhocko@...nel.org, ard.biesheuvel@...aro.org,
        sam@...nborg.org, mgorman@...hsingularity.net,
        Steve Sistare <steven.sistare@...cle.com>,
        daniel.m.jordan@...cle.com, bob.picco@...cle.com
Subject: Re: [PATCH v9 09/12] mm/kasan: kasan specific map populate function

On Mon, Oct 09, 2017 at 02:42:32PM -0400, Pavel Tatashin wrote:
> Hi Will,
> 
> In addition to what Michal wrote:
> 
> > As an interim step, why not introduce something like
> > vmemmap_alloc_block_flags and make the page-table walking opt-out for
> > architectures that don't want it? Then we can just pass __GFP_ZERO from
> > our vmemmap_populate where necessary and other architectures can do the
> > page-table walking dance if they prefer.
> 
> I do not see the benefit, implementing this approach means that we
> would need to implement two table walks instead of one: one for x86,
> another for ARM, as these two architectures support kasan. Also, this
> would become a requirement for any future architecture that want to
> add kasan support to add this page table walk implementation.

We have two table walks even with your patch series applied afaict: one in
our definition of vmemmap_populate (arch/arm64/mm/mmu.c) and this one
in the core code.

> >> IMO, while I understand that it looks strange that we must walk page
> >> table after creating it, it is a better approach: more enclosed as it
> >> effects kasan only, and more universal as it is in common code.
> >
> > I don't buy the more universal aspect, but I appreciate it's subjective.
> > Frankly, I'd just sooner not have core code walking early page tables if
> > it can be avoided, and it doesn't look hard to avoid it in this case.
> > The fact that you're having to add pmd_large and pud_large, which are
> > otherwise unused in mm/, is an indication that this isn't quite right imo.
> 
>  28 +#define pmd_large(pmd)         pmd_sect(pmd)
>  29 +#define pud_large(pud)         pud_sect(pud)
> 
> it is just naming difference, ARM64 calls them pmd_sect, common mm and
> other arches call them
> pmd_large/pud_large. Even the ARM has these defines in
> 
> arm/include/asm/pgtable-3level.h
> arm/include/asm/pgtable-2level.h

My worry is that these are actually highly arch-specific, but will likely
grow more users in mm/ that assume things for all architectures that aren't
necessarily valid.

Will