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Subject: Re: [PATCH 1/2] mm: use kvzalloc for swap_info_struct allocation
References: <37b60523-d085-71e9-fef9-80b90bfcef18@virtuozzo.com>
 <87wopsbb5v.fsf@yhuang-dev.intel.com>
From: Vasily Averin <vvs@virtuozzo.com>
Message-ID: <f702278c-4e2f-a7fd-0e0a-150284ec8cc1@virtuozzo.com>
Date: Mon, 5 Nov 2018 07:59:13 +0300
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Sender: owner-linux-mm@kvack.org
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To: "Huang, Ying" <ying.huang@intel.com>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org, Aaron Lu <aaron.lu@intel.com>



On 11/5/18 3:50 AM, Huang, Ying wrote:
> Vasily Averin <vvs@virtuozzo.com> writes:
> 
>> commit a2468cc9bfdf ("swap: choose swap device according to numa node")
>> increased size of swap_info_struct up to 44 Kbytes, now it requires
>> 4th order page.
> 
> Why swap_info_struct could be so large?  Because MAX_NUMNODES could be
> thousands so that 'avail_lists' field could be tens KB?  If so, I think
> it's fair to use kvzalloc().  Can you add one line comment?  Because
> struct swap_info_struct is quite small in default configuration.

I was incorrect not 44Kb but 40kb should be here.
We have found CONFIG_NODES_SHIFT=10 in new RHEL7 update 6 kernel,
default ubuntu kernels have the same setting too.

crash> struct swap_info_struct -o
struct swap_info_struct {
      [0] unsigned long flags;
      [8] short prio;
           ...
    [140] spinlock_t lock;
    [144] struct plist_node list;
    [184] struct plist_node avail_lists[1024]; <<<< here
  [41144] struct swap_cluster_info *cluster_info;
  [41152] struct swap_cluster_list free_clusters;
          ...
  [41224] spinlock_t cont_lock;
}
SIZE: 41232

struct swap_info_struct {
        ...
        RH_KABI_EXTEND(struct plist_node avail_lists[MAX_NUMNODES]) /* entry in swap_avail_head */
        ...
}

#define MAX_NUMNODES    (1 << NODES_SHIFT)

#ifdef CONFIG_NODES_SHIFT 
#define NODES_SHIFT     CONFIG_NODES_SHIFT
#else
#define NODES_SHIFT     0
#endif

/boot/config-4.15.0-38-generic:CONFIG_NODES_SHIFT=10