linux-kernel - Re: [PATCHv4 08/10] mm/kasan: Switch to using __pa_symbol and lm

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Date:   Tue, 6 Dec 2016 17:25:12 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Laura Abbott <labbott@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com
Subject: Re: [PATCHv4 08/10] mm/kasan: Switch to using __pa_symbol and
 lm_alias

On Thu, Dec 01, 2016 at 02:36:05PM +0300, Andrey Ryabinin wrote:
> On 11/29/2016 09:55 PM, Laura Abbott wrote:
> > __pa_symbol is the correct API to find the physical address of symbols.
> > Switch to it to allow for debugging APIs to work correctly.
> 
> But __pa() is correct for symbols. I see how __pa_symbol() might be a little
> faster than __pa(), but there is nothing wrong in using __pa() on symbols.

While it's true today that __pa() works on symbols, this is for
pragmatic reasons (allowing existing code to work until it is all
cleaned up), and __pa_symbol() is the correct API to use.

Relying on this means that __pa() can't be optimised for the (vastly
common) case of translating linear map addresses.

Consistent use of __pa_symbol() will allow for subsequent optimisation
of __pa() in the common case, in adition to being necessary for arm64's
DEBUG_VIRTUAL.

> > Other functions such as p*d_populate may call __pa internally.
> > Ensure that the address passed is in the linear region by calling
> > lm_alias.
> 
> Why it should be linear mapping address? __pa() translates kernel
> image address just fine.

As above, while that's true today, but is something that we wish to
change. 

> Generated code is probably worse too.

Even if that is the case, given this is code run once at boot on a debug
build, I think it's outweighed by the gain from DEBUG_VIRTUAL, and as a
step towards optimising __pa().

Thanks,
Mark.