Re: [patch -mmotm] mm: invoke oom killer for __GFP_NOFAIL

[Mel Gorman <mel@csn.ul.ie>]

On Mon, May 11, 2009 at 08:21:21PM +0900, Minchan Kim wrote:
> On Mon, May 11, 2009 at 6:12 PM, Minchan Kim <minchan.kim@gmail.com> wrote:
> > On Mon, May 11, 2009 at 5:40 PM, David Rientjes <rientjes@google.com> wrote:
> >> On Mon, 11 May 2009, Minchan Kim wrote:
> >>
> >>> Hmm.. if __alloc_pages_may_oom fail to allocate free page due to order > PAGE_ALLOC_COSTRY_ORDER,
> >>>
> >>> It will go to nopage label in __alloc_pages_slowpath.
> >>> Then it will show the page allocation failure warning and will return.
> >>> Retrying depends on caller.
> >>>
> >>
> >> Correct.
> >>
> >>> So, I think it won't loop forever.
> >>> Do I miss something ?
> >>>
> >>
> >> __GFP_NOFAIL allocations shouldn't fail, that's the point of the gfp flag.
> >> So failing without attempting to free some memory is the wrong thing to
> >> do.
> >
> > Thanks for quick reply.
> > I was confused by your description.
> > I thought you suggested we have to prevent loop forever.
> >
> >>
> >>> In addition, the OOM killer can help for getting the high order pages ?
> >>>
> >>
> >> Sure, if it selects a task that will free a lot of memory, which is it's
> >> goal.
> >>
> >
> > How do we know any task have a lot of memory ?
> > If we select wrong task and kill one ?
> >
> > I have a concern about innocent task.
> 
> Now, I look over __out_of_memory.
> For selecting better tasks in case of PAGE_ALLOC_COSTRY_ORDER, How
> about increasing score of task which have VM_HUGETLB vma in badness ?
> 

That is unjustified. It penalises a process even if it only allocated one
hugepage and it is not a reflection of how much memory the process is using
or how badly behaved it is.

Even worse, if the huge page was allocated from the static hugepage pool then
the hugepages are freed to the hugepage pool and not the page allocator when
the process is killed. This means that killing a process using hugepages
does not necessarily help applications requiring more memory unless they
also want hugepages. However, a hugepage allocation will not trigger the
OOM killer so killing processes using hugepages still does not help.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab