Re: linux-next: Tree for Nov 7

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:

quoted

On Fri, Nov 10, 2017 at 11:00 PM, Michal Hocko [off-list ref] wrote:

quoted

Hi Joel,

On Wed 08-11-17 15:20:50, Michal Hocko wrote:
[...]

quoted

quoted

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@dhcp22.suse.cz

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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