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

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Message-ID: <alpine.LRH.2.00.1808312312480.14475@gjva.wvxbf.pm>
Date:   Fri, 31 Aug 2018 23:18:09 +0200 (CEST)
From:   Jiri Kosina <jikos@...nel.org>
To:     Juergen Gross <jgross@...e.com>
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 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.

Thanks,

-- 
Jiri Kosina
SUSE Labs