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Date: Thu, 20 Dec 2018 12:35:47 +0100
From: Michal Hocko <mhocko@kernel.org>
To: Pingfan Liu <kernelfans@gmail.com>
Subject: Re: [PATCHv2 2/3] mm/numa: build zonelist when alloc for device on
 offline node
Message-ID: <20181220113547.GC9104@dhcp22.suse.cz>
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Cc: "H. Peter Anvin" <hpa@zytor.com>, Paul Mackerras <paulus@samba.org>,
 Ingo Molnar <mingo@redhat.com>, x86@kernel.org, linux-kernel@vger.kernel.org,
 linux-mm@kvack.org, Mike Rapoport <rppt@linux.vnet.ibm.com>,
 Borislav Petkov <bp@alien8.de>, Jonathan Cameron <Jonathan.Cameron@huawei.com>,
 Bjorn Helgaas <bhelgaas@google.com>, David Rientjes <rientjes@google.com>,
 Andrew Morton <akpm@linux-foundation.org>, linuxppc-dev@lists.ozlabs.org,
 Thomas Gleixner <tglx@linutronix.de>, Vlastimil Babka <vbabka@suse.cz>
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On Thu 20-12-18 17:50:38, Pingfan Liu wrote:
[...]
> @@ -453,7 +456,12 @@ static inline int gfp_zonelist(gfp_t flags)
>   */
>  static inline struct zonelist *node_zonelist(int nid, gfp_t flags)
>  {
> -	return NODE_DATA(nid)->node_zonelists + gfp_zonelist(flags);
> +	if (unlikely(!possible_zonelists[nid])) {
> +		WARN_ONCE(1, "alloc from offline node: %d\n", nid);
> +		if (unlikely(build_fallback_zonelists(nid)))
> +			nid = first_online_node;
> +	}
> +	return possible_zonelists[nid] + gfp_zonelist(flags);
>  }

No, please don't do this. We do not want to make things work magically
and we definitely do not want to put something like that into the hot
path. We definitely need zonelists to be build transparently for all
possible nodes during the init time.
-- 
Michal Hocko
SUSE Labs