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

[Vlastimil Babka <vbabka@suse.cz>]

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?

> 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.

> 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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