linux-kernel - Re: [PATCH 1/2] mm: use kvzalloc for swap_info

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Date:   Mon, 5 Nov 2018 14:10:16 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     Vasily Averin <vvs@...tuozzo.com>
Cc:     "Huang, Ying" <ying.huang@...el.com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: use kvzalloc for swap_info_struct allocation

On Mon, Nov 05, 2018 at 07:59:13AM +0300, Vasily Averin wrote:
> 
> 
> On 11/5/18 3:50 AM, Huang, Ying wrote:
> > Vasily Averin <vvs@...tuozzo.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

So every 'struct plist_node' takes 40 bytes and 1024 of them take a
total of 40960 bytes, which is 10 pages and need an order-4 page to host
them. It looks a little too much, especially consider most of the space
will left be unused since most systems have nodes <= 4. I didn't realize
this problem when developing this patch, thanks for pointing this out.

I think using kvzalloc() as is done by your patch is better here as it
can avoid possible failure of swapon.

Acked-by: Aaron Lu <aaron.lu@...el.com>

BTW, for systems with few swap devices this may not be a big deal, but
according to your description, your workload will create a lot of swap
devices and each of them will likely cause an order-4 unmovable pages
allocated(when kvzalloc() doesn't fallback). I was thinking maybe we
should convert avail_lists to a pointer in swap_info_struct and use
vzalloc() for it.

Thanks,
Aaron

>   [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
>