Re: linux-next: Tree for Nov 7

[Russell King - ARM Linux]

On Mon, Nov 13, 2017 at 10:20:06AM +0100, Michal Hocko wrote:
> [Cc arm and ppc maintainers]
> 
> Thanks a lot for testing!
> 
> On Sun 12-11-17 11:38:02, Joel Stanley wrote:
> > On Fri, Nov 10, 2017 at 11:00 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > > Hi Joel,
> > >
> > > On Wed 08-11-17 15:20:50, Michal Hocko wrote:
> > > [...]
> > >> > There are a lot of messages on the way up that look like this:
> > >> >
> > >> > [    2.527460] Uhuuh, elf segement at 000d9000 requested but the
> > >> > memory is mapped already
> > >> > [    2.540160] Uhuuh, elf segement at 000d9000 requested but the
> > >> > memory is mapped already
> > >> > [    2.546153] Uhuuh, elf segement at 000d9000 requested but the
> > >> > memory is mapped already
> > >> >
> > >> > And then trying to run userspace looks like this:
> > >>
> > >> Could you please run with debugging patch posted
> > >> http://lkml.kernel.org/r/20171107102854.vylrtaodla63kc57@xxxxxxxxxxxxxx
> > >
> > > Did you have chance to test with this debugging patch, please?
> > 
> > Lots of this:
> > 
> > [    1.177266] Uhuuh, elf segement at 000d9000 requested but the  memory is mapped already, got 000dd000
> > [    1.177555] Clashing vma [dd000, de000] flags:100873 name:(null)
> 
> This smells like the problem I've expected that mmap with hint doesn't
> respect the hint even though there is no clashing mapping. The above
> basically says that we didn't map at 0xd9000 but it has placed it at
> 0xdd000. The nearest (clashing) vma is at 0xdd000 so this is our new
> mapping. find_vma returns the closest vma (with addr < vm_end) for the
> given address 0xd9000 so this address cannot be mapped by any other vma.
> 
> Now that I am looking at arm's arch_get_unmapped_area it does perform
> aligning for shared vmas. We do not do that for MAP_FIXED.  Powepc,
> reported earlier [1] seems to suffer from the similar problem.
> slice_get_unmapped_area alignes to slices, whatever that means.
> 
> I can see two possible ways around that. Either we explicitly request
> non-aligned mappings via a special MAP_$FOO (e.g. MAP_FIXED_SAFE) or
> simply opt out from the MAP_FIXED protection via ifdefs. The first
> option sounds more generic to me but also more tricky to not introduce
> other user visible effects. The later is quite straightforward. What do
> you think about the following on top of the previous patch?
> 
> It is rather terse and disables the MAP_FIXED protection for arm
> comletely because I couldn't find a way to make it conditional on
> CACHEID_VIPT_ALIASING. But this can be always handled later. I find the
> protection for other archtectures useful enough to have this working for
> most architectures now and handle others specially.

Can someone provide the background information for this please?

-- 
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