Re: [RFC PATCH 0/2] mm: fix OOMs for binding workloads to movable zone only node

[Feng Tang]

On Thu, Nov 05, 2020 at 05:16:12PM +0100, Michal Hocko wrote:
> On Thu 05-11-20 21:43:05, Feng Tang wrote:
> > On Thu, Nov 05, 2020 at 02:12:45PM +0100, Michal Hocko wrote:
> > > On Thu 05-11-20 21:07:10, Feng Tang wrote:
> > > [...]
> > > > My debug traces shows it is, and its gfp_mask is 'GFP_KERNEL'
> > > 
> > > Can you provide the full information please? Which node has been
> > > requested. Which cpuset the calling process run in and which node has
> > > the allocation succeeded from? A bare dump_stack without any further
> > > context is not really helpful.
> > 
> > I don't have the same platform as the original report, so I simulated
> > one similar setup (with fakenuma and movablecore), which has 2 memory
> > nodes: node 0 has DMA0/DMA32/Movable zones, while node 1 has only
> > Movable zone. With it, I can got the same error and same oom callstack
> > as the original report (as in the cover-letter).
> > 
> > The test command is:
> > 	# docker run -it --rm --cpuset-mems 1 ubuntu:latest bash -c "grep Mems_allowed /proc/self/status"
> > 
> > To debug I only added some trace in the __alloc_pages_nodemask(), and
> > for the callstack which get the page successfully:
> > 
> > 	[  567.510903] Call Trace:
> > 	[  567.510909]  dump_stack+0x74/0x9a
> > 	[  567.510910]  __alloc_pages_nodemask.cold+0x22/0xe5
> > 	[  567.510913]  alloc_pages_current+0x87/0xe0
> > 	[  567.510914]  __vmalloc_node_range+0x14c/0x240
> > 	[  567.510918]  module_alloc+0x82/0xe0
> > 	[  567.510921]  bpf_jit_alloc_exec+0xe/0x10
> > 	[  567.510922]  bpf_jit_binary_alloc+0x7a/0x120
> > 	[  567.510925]  bpf_int_jit_compile+0x145/0x424
> > 	[  567.510926]  bpf_prog_select_runtime+0xac/0x130
> 
> As already said this doesn't really tell much without the additional
> information.
> 
> > The incomming parameter nodemask is NULL, and the function will first try the
> > cpuset nodemask (1 here), and the zoneidx is only granted 2, which makes the
> > 'ac's preferred zone to be NULL. so it goes into __alloc_pages_slowpath(),
> > which will first set the nodemask to 'NULL', and this time it got a preferred
> > zone: zone DMA32 from node 0, following get_page_from_freelist will allocate
> > one page from that zone. 
> 
> I do not follow. Both hot and slow paths of the allocator set
> ALLOC_CPUSET or emulate it by mems_allowed when cpusets are nebaled
> IIRC. This is later enforced in get_page_from_free_list. There are some
> exceptions when the allocating process can run away from its cpusets -
> e.g. IRQs, OOM victims and few other cases but definitely not a random
> allocation. There might be some subtle details that have changed or I
> might have forgot but 

yes, I was confused too. IIUC, the key check inside get_page_from_freelist()
is 

	if (cpusets_enabled() &&
		(alloc_flags & ALLOC_CPUSET) &&
		!__cpuset_zone_allowed(zone, gfp_mask))

In our case (kernel page got allocated), the first 2 conditions are true,
and for __cpuset_zone_allowed(), the possible place to return true is
checking parent cpuset's nodemask

	cs = nearest_hardwall_ancestor(task_cs(current));
	allowed = node_isset(node, cs->mems_allowed);

This will override the ALLOC_CPUSET check.

Thanks,
Feng
> -- 
> Michal Hocko
> SUSE Labs