Re: [RFC] can we use vmalloc to alloc thread stack if compaction failed

[Michal Hocko]

On Thu 28-07-16 16:45:06, Xishi Qiu wrote:
> On 2016/7/28 15:58, Michal Hocko wrote:
> 
> > On Thu 28-07-16 15:41:53, Xishi Qiu wrote:
> >> On 2016/7/28 15:20, Michal Hocko wrote:
> >>
> >>> On Thu 28-07-16 15:08:26, Xishi Qiu wrote:
> >>>> Usually THREAD_SIZE_ORDER is 2, it means we need to alloc 16kb continuous
> >>>> physical memory during fork a new process.
> >>>>
> >>>> If the system's memory is very small, especially the smart phone, maybe there
> >>>> is only 1G memory. So the free memory is very small and compaction is not
> >>>> always success in slowpath(__alloc_pages_slowpath), then alloc thread stack
> >>>> may be failed for memory fragment.
> >>>
> >>> Well, with the current implementation of the page allocator those
> >>> requests will not fail in most cases. The oom killer would be invoked in
> >>> order to free up some memory.
> >>>
> >>
> >> Hi Michal,
> >>
> >> Yes, it success in most cases, but I did have seen this problem in some
> >> stress-test.
> >>
> >> DMA free:470628kB, but alloc 2 order block failed during fork a new process.
> >> There are so many memory fragments and the large block may be soon taken by
> >> others after compact because of stress-test.
> >>
> >> --- dmesg messages ---
> >> 07-13 08:41:51.341 <4>[309805.658142s][pid:1361,cpu5,sManagerService]sManagerService: page allocation failure: order:2, mode:0x2000d1
> > 
> > Yes but this is __GFP_DMA allocation. I guess you have already reported
> > this failure and you've been told that this is quite unexpected for the
> > kernel stack allocation. It is your out-of-tree patch which just makes
> > things worse because DMA restricted allocations are considered "lowmem"
> > and so they do not invoke OOM killer and do not retry like regular
> > GFP_KERNEL allocations.
> 
> Hi Michal,
> 
> Yes, we add GFP_DMA, but I don't think this is the key for the problem.

You are restricting the allocation request to a single zone which is
definitely not good. Look at how many larger order pages are available
in the Normal zone.

> If we do oom-killer, maybe we will get a large block later, but there
> is enough free memory before oom(although most of them are fragments).

Killing a task is of course the last resort action. It would give you
larger order blocks used for the victims thread.

> I wonder if we can alloc success without kill any process in this situation.

Sure it would be preferable to compact that memory but that might be
hard with your restriction in place. Consider that DMA zone would tend
to be less movable than normal zones as users would have to pin it for
DMA. Your DMA is really large so this might turn out to just happen to
work but note that the primary problem here is that you put a zone
restriction for your allocations.

> Maybe use vmalloc is a good way, but I don't know the influence.

You can have a look at vmalloc patches posted by Andy. They are not that
trivial.

-- 
Michal Hocko
SUSE Labs