Re: alloc_pages_exact() triggering memory fragmentation on nommu?

[Nikita Yushchenko]

> I'm having a guess that this can be caused by use of
> alloc_pages_exact() for NoMMU private anonymous mappings.
> 
> This routine causes "tail" of allocation to be returned back
> to allocator... and inserted at top of free list. Later, when
> whatever in the system makes a trivial order-0 allocation, these
> just-freed tails immediately get used (because free pages are
> inserted at end of free lists... and new pages are allocated
> from either beginning or end of free list, depending on 'cold'
> parameter).  At a glance, this should have a net effect of much
> increased probability of "tail" of large allocation get used
> as a small allocation, and thus inability to rebuild a large free
> block at time when large allocation is freed.
> 
> Is there any protection against this effect in the allocator
> of current kernels? (kernel of system in question is somewhat
> outdated)

Just a small followup to whoever may be interested.

The guess was correct.

In old 3.0.8 kernel running on system in question, do_mmap_private() in
mm/nommu.c did not yet use alloc_pages_exact() but instead had it's own
implementation of the same logic.  And there was a tunable
'nr_trim_pages' that could alter it. In particular setting that tunable
to 0 effectively disabled any freeing of tails.

We tried to set nr_trim_pages=0 and fragmentation issue went away.


Nikita