Re: [PATCH] mem-hotplug: Don't clear the only node in new_node_page()

[Li Zhong <zhong@linux.vnet.ibm.com>]

> On Sep 5, 2016, at 22:18, Vlastimil Babka <vbabka@suse.cz> wrote:
> 
> On 09/05/2016 04:59 AM, Li Zhong wrote:
>> Commit 394e31d2c introduced new_node_page() for memory hotplug.
>> 
>> In new_node_page(), the nid is cleared before calling __alloc_pages_nodemask().
>> But if it is the only node of the system,
> 
> So the use case is that we are partially offlining the only online node?

Yes.
> 
>> and the first round allocation fails,
>> it will not be able to get memory from an empty nodemask, and trigger oom.
> 
> Hmm triggering OOM due to empty nodemask sounds like a wrong thing to do. CCing some OOM experts for insight. Also OOM is skipped for __GFP_THISNODE allocations, so we might also consider the same for nodemask-constrained allocations?
> 
>> The patch checks whether it is the last node on the system, and if it is, then
>> don't clear the nid in the nodemask.
> 
> I'd rather see the allocation not OOM, and rely on the fallback in new_node_page() that doesn't have nodemask. But I suspect it might also make sense to treat empty nodemask as something unexpected and put some WARN_ON (instead of OOM) in the allocator.

I think it would be much easier to understand these kind of empty nodemask allocation failure with this WARN_ON(), how about something like this?

===
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a2214c6..57edf18 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3629,6 +3629,11 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
                .migratetype = gfpflags_to_migratetype(gfp_mask),
        };
 
+       if (nodemask && nodes_empty(*nodemask)) {
+               WARN_ON(1);
+               return NULL;
+       }
+
        if (cpusets_enabled()) {
                alloc_mask |= __GFP_HARDWALL;
                alloc_flags |= ALLOC_CPUSET;
===

If that’s ok, maybe I can send a separate patch for this? 

Thanks, Zhong

> 
>> Reported-by: John Allen <jallen@linux.vnet.ibm.com>
>> Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Fixes: 394e31d2ceb4 ("mem-hotplug: alloc new page from a nearest neighbor node when mem-offline")
> 
>> ---
>> mm/memory_hotplug.c | 4 +++-
>> 1 file changed, 3 insertions(+), 1 deletion(-)
>> 
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 41266dc..b58906b 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -1567,7 +1567,9 @@ static struct page *new_node_page(struct page *page, unsigned long private,
>> 		return alloc_huge_page_node(page_hstate(compound_head(page)),
>> 					next_node_in(nid, nmask));
>> 
>> -	node_clear(nid, nmask);
>> +	if (nid != next_node_in(nid, nmask))
>> +		node_clear(nid, nmask);
>> +
>> 	if (PageHighMem(page)
>> 	    || (zone_idx(page_zone(page)) == ZONE_MOVABLE))
>> 		gfp_mask |= __GFP_HIGHMEM;
>> 
>> 
>> 
> 

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