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Date:   Fri, 31 Aug 2018 23:18:09 +0200 (CEST)
From:   Jiri Kosina <>
To:     Juergen Gross <>
cc:     Linux Kernel Mailing List <>,
        Linus Torvalds <>,
        Michal Hocko <>
Subject: Re: Access to non-RAM pages

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.


Jiri Kosina

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