linux-kernel - Re: Access to non-RAM pages

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Message-ID: <b1b5ef64-d49b-e3f7-98b4-0ae3fd57b812@suse.com>
Date:   Sat, 1 Sep 2018 12:47:48 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jiri Kosina <jikos@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: Access to non-RAM pages

On 31/08/18 23:18, Jiri Kosina wrote:
> On Wed, 29 Aug 2018, Juergen Gross wrote:
> 
>> While being very unlikely I still believe this is possible. Any
>> thoughts?
> 
> So in theory we should somehow test whether the next page is some form of 
> mmio/gart/... mapping, but I guess that by itself would kill the 
> performance advantage of the whole load_unaligned_zeropad() trick.
> 
> And yes, the sideffects of reading from mmio mapping is a real thing -- 
> see for example the issue fixed by 2a3e83c6f9.
> 
> If noone has any clever idea how to work this around (I don't), I am 
> afraid we'd have to ditch the whole DCACHE_WORD_ACCESS optimization, as 
> it's silently dangerous.

Are there any architectures which can still use DCACHE_WORD_ACCESS?

x86 shouldn't, what about powerpc, arm and arm64?

I'm happy to send patches to remove selection of DCACHE_WORD_ACCESS
and eventually to remove all of the load_unaligned_zeropad() stuff.


Juergen