Messages in this thread | | | Date | Tue, 1 Dec 2015 12:23:41 +0000 | From | Will Deacon <> | Subject | Re: [lkp] [mm, page_alloc] d0164adc89: -100.0% fsmark.app_overhead |
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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.
> 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...
Will
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