Re: [Lse-tech] Re: Hugetlbpages in very large memory machines.......

[Anton Blanchard]

> Demand-paging the hugepages is a decent feature to have, and ISTR resisting
> it before for this reason.
> 
> Even though it's early in the 2.6 series I'd be a bit worried about
> breaking existing hugetlb users in this way.  Yes, the pages are
> preallocated so it is unlikely that a working setup is suddenly going to
> break.  Unless someone is using the return value from mmap to find out how
> many pages they can get.

Hmm what a coincidence, I was chasing a problem where large page
allocations would fail even though I clearly had enough large page memory
free.

It turns out we were tripping the overcommit logic in do_mmap. I had
30GB of large page and 2GB of small pages and of course cap_vm_enough_memory
was looking at the small page pool. Setting overcommit to 1 fixed it.

It seems we can solve both problems by having a separate hugetlb overcommit
policy. Make it strict and you wont have OOM problems on large pages
and I wont hit my 30GB / 2GB problem.

Anton
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