Re: [RFC] Warn the user when they could overflow mapcount

[Matthew Wilcox]

On Thu, Feb 08, 2018 at 02:48:52PM -0500, Daniel Micay wrote:
> I guess it could saturate and then switch to tracking the count via an
> object pointer -> count mapping with a global lock? Whatever the
> solution is should probably be a generic one since it's a recurring
> issue.

I was thinking of saturating _mapcount at 2 billion (allowing _refcount
the extra space to go into the 2-3 billion range).  Once saturated,
disallow all attempts at mapping it until _mapcount has gone below 2
billion again.  We can walk the page->mapping->i_mmap tree and find
tasks with more than, say, 10 mappings each, and kill them.

Now that I think about it, though, perhaps the simplest solution is not
to worry about checking whether _mapcount has saturated, and instead when
adding a new mmap, check whether this task already has it mapped 10 times.
If so, refuse the mapping.

Now we can argue that since pid_max is smaller than 400 million that
_mapcount will never overflow, and so we don't need to check it.
Convincing argument?