Re: [lkp] [mm, page_alloc] d0164adc89: -100.0% fsmark.app_overhead

[Mel Gorman <mgorman@techsingularity.net>]

On Tue, Dec 01, 2015 at 03:04:31PM +0100, Michal Hocko wrote:
> On Tue 01-12-15 12:23:41, Will Deacon wrote:
> > Hi Michal,
> > 
> > On Mon, Nov 30, 2015 at 02:02:00PM +0100, Michal Hocko wrote:
> > > [Let's CC Will - see the question at the end of the email please]
> > 
> > [...]
> > 
> > > > > There is no reference to OOM possibility in the email that I can see. Can
> > > > > you give examples of the OOM messages that shows the problem sites? It was
> > > > > suspected that there may be some callers that were accidentally depending
> > > > > on access to emergency reserves. If so, either they need to be fixed (if
> > > > > the case is extremely rare) or a small reserve will have to be created
> > > > > for callers that are not high priority but still cannot reclaim.
> > > 
> > > __virtblk_add_req calls
> > > virtqueue_add_sgs(vq, sgs, num_out, num_in, vbr, GFP_ATOMIC)
> > >   alloc_indirect(gfp)
> > >     gfp &= ~(__GFP_HIGHMEM | __GFP_HIGH)
> > > 
> > > So this is true __GFP_ATOMIC, we just drop __GFP_HIGH so it doesn't get
> > > access to more reserves. It still does ALLOC_HARDER. So I think the real
> > > issue is somewhere else when something should have triggered kswapd and
> > > it doesn't do that anymore. I have tried to find that offender the last
> > > time but didn't manage to find any.
> > > 
> > > Btw. I completely miss why b92b1b89a33c ("virtio: force vring
> > > descriptors to be allocated from lowmem") had to clear __GFP_HIGH. Will
> > > do you remember why you have dropped that flag as well?
> > 
> > Right, that looks unnecessary, but it could be that we were masking a
> > bug somewhere else.
> 
> OK, I will send a patch to remove __GFP_HIGH because it is clearly
> misleding and doesn't have anything to do with the highmem zone.
> 

Thanks for looking into this. I just sent a patch that includes a changelog
explaining why this bug triggers now and would have been hidden before.

> > > Also I do not seem to find any user of alloc_indirect which would do
> > > __GFP_HIGHMEM. All of them are either GFP_KERNEL or GFP_ATOMIC. So
> > > either I am missing something or this is not really needed. Maybe the
> > > situation was different back in 2012.
> > 
> > I tried to revisit the thread leading to that patch, but it doesn't make
> > a whole lot of sense:
> > 
> >   https://lkml.org/lkml/2012/10/17/143
> > 
> > I certainly remember debugging the failure (i.e. it wasn't theoretical),
> > and we were ending up with highmem addresses being passed in the virtio
> > ring (due to the zero-copy stuff in 9p) and also for the descriptors
> > themselves. The discussion at the time makes it sound like GFP_ATOMIC
> > was giving us those...
> 
> Hmm, unless I am missing something GFP_ATOMIC resp. GFP_KERNEL cannot
> fallback to the highmem zone - see GFP_ZONE_TABLE. Maybe the highmem
> pointer got there from a different path than alloc_indirect?

GFP_ATOMIC should not be returning highmem addresses ever. I checked
briefly but did not spot where 9p is getting highmem pages from but it
wasn't via GFP_ATOMIC.

-- 
Mel Gorman
SUSE Labs