Re: linux-next: Tree for Nov 7

[Russell King - ARM Linux]

On Mon, Nov 13, 2017 at 03:11:40PM +0100, Michal Hocko wrote:
> On Mon 13-11-17 10:20:06, 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.
> 
> Sorry for confusion here. These are not shared mappings as pointed out
> by Russell in a private email. I got confused by the above flags which I
> have misinterpreted as bit 0 set => MAP_SHARED. These are vm_flags
> obviously so the bit 0 is VM_READ. Sorry about the confusion. The real
> reason we are doing the alignment is that we do a file mapping
> 	/*
> 	 * We only need to do colour alignment if either the I or D
> 	 * caches alias.
> 	 */
> 	if (aliasing)
> 		do_align = filp || (flags & MAP_SHARED);
> 
> I am not really familiar with this architecture to understand why do we
> need aliasing for file mappings, though.

I think it's there so that flush_dcache_page() works - possibly
get_user_pages() being used on a private mapping of page cache pages,
but that's guessing.

I'm afraid I don't remember all the details, this is code from around
15 years ago, and I'd be very nervous about changing it now without
fully understanding the issues.

-- 
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