linux-kernel - Re: linux-next: Tree for Nov 7

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Date:   Mon, 13 Nov 2017 13:00:57 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Joel Stanley <joel@....id.au>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Russell King <linux@...linux.org.uk>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Abdul Haleem <abdhalee@...ux.vnet.ibm.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        "James E.J. Bottomley" <jejb@...isc-linux.org>,
        Helge Deller <deller@....de>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-mips@...ux-mips.org,
        linux-parisc@...r.kernel.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org
Subject: Re: linux-next: Tree for Nov 7

On Mon 13-11-17 22:34:50, Michael Ellerman wrote:
> Hi Michal,
> 
> Michal Hocko <mhocko@...nel.org> writes:
> > On Mon 13-11-17 10:20:06, Michal Hocko wrote:
> >> [Cc arm and ppc maintainers]
> >
> > Hmm, it turned out to be a problem on other architectures as well.
> > CCing more maintainers. For your reference, we are talking about
> > http://lkml.kernel.org/r/20171023082608.6167-1-mhocko@kernel.org
> > which has broken architectures which do apply aligning on the mmap
> > address hint without MAP_FIXED applied. See below my proposed way
> > around this issue because I belive that the above patch is quite
> > valuable on its own to be dropped for all archs.
> 
> I don't really like your solution sorry :)  The fact that you've had to
> patch seven arches seems like a red flag.
> 
> I think this is a generic problem with MAP_FIXED, which I've heard
> userspace folks complain about in the past.

The thing is that we canno  change MAP_FIXED behavior as it is carved in
stone

> Currently MAP_FIXED does two things:
>   1. makes addr not a hint but the required address
>   2. blasts any existing mapping
> 
> You want 1) but not 2).

+ fail if there is a clashing range

> So the right solution IMHO would be to add a new mmap flag to request
> that behaviour, ie. a fixed address but iff there is nothing already
> mapped there.
> 
> I don't know the mm code well enough to know if that's hard for some
> reason, but it *seems* like it should be doable.

Yes, I have mentioned that in the previous email but the amount of code
would be even larger. Basically every arch which reimplements
arch_get_unmapped_area would have to special case new MAP_FIXED flag to
do vma lookup. So this was the most simple solution I could come up
with. If there was a general interest for MAP_FIXED_SAFE then we can
introduce it later of course. I would just like the hardening merged
sooner rather than later.
-- 
Michal Hocko
SUSE Labs