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

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Date:   Mon, 13 Nov 2017 22:34:50 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Michal Hocko <mhocko@...nel.org>, Joel Stanley <joel@....id.au>
Cc:     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

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.

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).

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.

cheers