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

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Message-ID: <d2f74ff960be5b1933ca3af35031b5a8156ff956.camel@kernel.crashing.org>
Date:   Mon, 03 Sep 2018 11:44:20 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jiri Kosina <jikos@...nel.org>,
        Jürgen Groß <jgross@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Will Deacon <will.deacon@....com>
Subject: Re: Access to non-RAM pages

On Sun, 2018-09-02 at 18:38 -0700, Linus Torvalds wrote:
> On Sun, Sep 2, 2018 at 6:32 PM Benjamin Herrenschmidt
> <benh@...nel.crashing.org> wrote:
> > 
> > Also, if we cross page boundaries with those guys then we have a bigger
> > problem no ? we could fall off a vmalloc page into the nether or into
> > an ioremap mapping no ?
> 
> It's not used for vmalloc stuff. It's just regular kmalloc().
> 
> So it can cross pages, and it can fall off the end of memory, but it
> can't do random stuff.

Ok, it might be worth adding a DEBUG_VM based (or similar) warning in
case somebody ever thinks of passing a vmalloc pointer to it...

As for falling out of the end of memory, yes it could be a real problem
though I don't see why IO is any different than just hitting a non-
mapped area in that regard. So we should probably keep an unused
(readonly if possible) zero page at the end.
 
Cheers,
Ben.