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

[Andrew Morton <akpm@osdl.org>]

Andy Whitcroft <apw@shadowen.org> wrote:
>
> --On 14 March 2004 15:06 +1100 Anton Blanchard <anton@samba.org> wrote:
> 
> > 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.
> 
> Been following this thread and it seems that fixing this overcommit
> miss-handling problem would logically be the first step.  From my reading
> it seems that once we have initialised hugetlb we have two independent and
> non-overlapping 'page' pools from which we can allocate pages and against
> which we wish to handle commitments.  Looking at the current code base we
> effectivly have only a single 'accounting domain' and so when we attempt to
> allocate hugetlb pages we incorrectly account them against the small page
> pool.
> 
> I believe we need to add support for more than one page 'accounting domain'
> each with its own policy and with its own commitments.  The attached patch
> is my attempt at this first step.  I have created the concept of an
> accounting domain, against which pages are to be accounted.  In this
> implementation there are two domains VM_AD_DEFAULT which is used to account
> normal small pages in the normal way and VM_AD_HUGETLB which is used to
> select and identify VM_HUGETLB pages.  I have not attempted to add any
> actual accounting for VM_HUGETLB pages, as currently they are prefaulted
> and thus there is always 0 outstanding commitment to track.  Obviously, if
> hugetlb was also changed to support demand paging that would need to be
> implemented.

Seems reasonable, although "vm_enough_acctdom" makes my eyes pop.  Why not
keep the "vm_enough_memory" identifier?

I've asked Stephen for comment - assuming he's OK with it I'd ask you to
finish this off please.