Re: reduce shrink_mmap rate of failure (initial attempt)

[Roger Larsson <roger.larsson@norran.net>]

Hi again,

There is an even more likely scenario...

* shrink_mmap is called when no zone is under pressure.
  then it will search the list over and over again...

Who could do such a nasty thing?

do_try_to_free_pages !

  Its agenda is not to stop when there is no pressure - it will go on
  until FREE_COUNT (8) pages are freed...

Even in the normal ac10 this should be a problem.
Several turns in do_try_to_free_pages with shrink_mmap failing will be
done - each swapping out some pages...

It will take time and swap...

/RogerL


Roger Larsson wrote:
> 
> >That patch hangs my machine here when I run mmap002.  The machine is
> >in shrink_mmap.  It hangs trying to get the pagmap_lru_lock.
> >
> >I think that the idea is good, but it doesn't work here :(.
> >
> >Later, Juan.
> 
> Ouch...
> 
> The only possible explaination is that we are searching for pages on a zone.
> But no such pages are possible to free from LRU...
> And we LOOP the list, holding the lru lock...
> Note: without this patch you may end up in another bad situation where
> shrink_mmap always fails and swapping will start until it swaps out a page
> of that specific zone.
> And without the test? We would free all other LRU pages without finding one
> that we want :-(
> 
> This will be interesting to fix...
> 
> May the allocation of pages play a part? Filling zone after zone will give no
> mix between the zones.
> 
> /RogerL
> (from work)
> 
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