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

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Date:   Mon, 9 Oct 2017 19:48:29 +0100
From:   Will Deacon <will.deacon@....com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Pavel Tatashin <pasha.tatashin@...cle.com>,
        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, 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 08:14:33PM +0200, Michal Hocko wrote:
> On Mon 09-10-17 13:51:47, Pavel Tatashin wrote:
> > I can go back to that approach, if Michal OK with it. But, that would
> > mean that I would need to touch every single architecture that
> > implements vmemmap_populate(), and also pass flags at least through
> > these functions on every architectures (some have more than one
> > decided by configs).:
> > 
> > vmemmap_populate()
> > vmemmap_populate_basepages()
> > vmemmap_populate_hugepages()
> > vmemmap_pte_populate()
> > __vmemmap_alloc_block_buf()
> > alloc_block_buf()
> > vmemmap_alloc_block()
> > 
> > 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.
> 
> While I understand that gfp mask approach might look better at first
> sight this is by no means a general purpose API so I would rather be
> pragmatic and have a smaller code footprint than a more general
> interface. Kasan is pretty much a special case and doing a one time
> initialization 2 pass thing is imho acceptable. If this turns out to be
> impractical in future then let's fix it up but right now I would rather
> go a simpler path.

I think the simpler path for arm64 is really to say when we want the memory
zeroing as opposed to exposing pmd_large/pud_large macros. Those are likely
to grow more users too, but are difficult to use correctly as we have things
like contiguous ptes that map to a granule smaller than a pmd.

I proposed an alternative solution to Pavel already, but it could be made
less general purpose by marking the function __meminit and only having it
do anything if KASAN is compiled in.

Will