Re: [PATCH v3] mm: prevent endless growth of anon_vma hierarchy

[Andrew Morton]

On Wed, 26 Nov 2014 15:05:59 -0600 Daniel Forrest <dan.forrest@xxxxxxxxxxxxx> wrote:

> On Wed, Nov 26, 2014 at 10:11:45PM +0400, Konstantin Khlebnikov wrote:
> 
> > Constantly forking task causes unlimited grow of anon_vma chain.
> > Each next child allocate new level of anon_vmas and links vmas to all
> > previous levels because it inherits pages from them. None of anon_vmas
> > cannot be freed because there might be pages which points to them.
> > 
> > This patch adds heuristic which decides to reuse existing anon_vma instead
> > of forking new one. It counts vmas and direct descendants for each anon_vma.
> > Anon_vma with degree lower than two will be reused at next fork.
> > 
> > As a result each anon_vma has either alive vma or at least two descendants,
> > endless chains are no longer possible and count of anon_vmas is no more than
> > two times more than count of vmas.
> 
> While I was working on the previous fix for this bug, Andrew Morton
> noticed that the error return from anon_vma_clone() was being dropped
> and replaced with -ENOMEM (which is not itself a bug because the only
> error return value from anon_vma_clone() is -ENOMEM).
> 
> I did an audit of callers of anon_vma_clone() and discovered an actual
> bug where the error return was being lost.  In __split_vma(), between
> Linux 3.11 and 3.12 the code was changed so the err variable is used
> before the call to anon_vma_clone() and the default initial value of
> -ENOMEM is overwritten.  So a failure of anon_vma_clone() will return
> success since err at this point is now zero.
> 
> Below is a patch which fixes this bug and also propagates the error
> return value from anon_vma_clone() in all cases.
> 
> I can send this as a separate patch, but maybe it would be easier if
> you were to incorporate it into yours?
> 

I grabbed it.  A bugfix is a bugfix.
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