Re: [Bug 67651] Bisected: Lots of fragmented mmaps cause gimp tofail in 3.12 after exceeding vm_max_map_count

[Cyrill Gorcunov]

On Wed, Jan 22, 2014 at 10:09:10PM -0800, Andrew Morton wrote:
> > > 
> > > That being said, this could cause vma blowups for programs that are
> > > actually using this thing.
> > 
> > Hi Andy, indeed, this could happen. The easiest way is to ignore softdirty bit
> > when we're trying to merge vmas and set it one new merged. I think this should
> > be correct. Once I finish I'll send the patch.
> 
> Hang on.  We think the problem is that gimp is generating vmas which
> *should* be merged, but for unknown reasons they differ in
> VM_SOFTDIRTY, yes?

Yes. One place where I forgot to set softdirty bit is setup_arg_pages. But
it called once on elf load, so it can't cause such effect (but should be
fixed too). Also there is do_brk where vmasoftdirty is missed too :/

Another problem is the potential scenario when we have a bunch of vmas
and clear vma-softdirty bit on them, then we try to map new one, flags
won't match and instead of extending old vma the new one will be created.
I think (if only I'm not missing something) that vma-softdirty should
be ignored in such case (ie inside is_mergeable_vma) and once vma extended
it should be marked as dirty one. Again, I need to think and test more.

> Shouldn't we work out where we're forgetting to set VM_SOFTDIRTY? 
> Putting bandaids over this error when we come to trying to merge the
> vmas sounds very wrong?

I'm looking into this as well.

	Cyrill
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