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    SubjectRe: [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

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