Re: [PATCH 1/2] virtio_balloon: disable oom killer when fill balloon

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> When memory pressure is high, virtio ballooning will probably cause oom killing.
> Even if alloc_page with GFP_NORETRY itself does not directly trigger oom it
> will make memory becoming low then memory alloc of other processes will trigger
> oom killing. It is not desired behaviour.
> 
> Here disable oom killer in fill_balloon to address this issue.
> 
> Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
> ---
>  drivers/virtio/virtio_balloon.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> --- linux-2.6.orig/drivers/virtio/virtio_balloon.c	2010-10-13 10:14:38.000000000 +0800
> +++ linux-2.6/drivers/virtio/virtio_balloon.c	2011-04-26 11:38:43.979785141 +0800
> @@ -25,6 +25,7 @@
>  #include <linux/freezer.h>
>  #include <linux/delay.h>
>  #include <linux/slab.h>
> +#include <linux/oom.h>
>  
>  struct virtio_balloon
>  {
> @@ -102,6 +103,7 @@ static void fill_balloon(struct virtio_b
>  	/* We can only do one array worth at a time. */
>  	num = min(num, ARRAY_SIZE(vb->pfns));
>  
> +	oom_killer_disable();

I think this patch need proper comment at least. My first impression
is, "Hm, __GFP_NORETRY should prevent oom, why is this necessary?".
So, this actually prevent _another_ thread call out_of_memory().
Also, Here doesn't have any exclusion against hibernation (ie another
oom_killer_disable() callsite). It should be described why lock is 
unnecessary.

Thanks.


>  	for (vb->num_pfns = 0; vb->num_pfns < num; vb->num_pfns++) {
>  		struct page *page = alloc_page(GFP_HIGHUSER | __GFP_NORETRY |
>  					__GFP_NOMEMALLOC | __GFP_NOWARN);
> @@ -119,6 +121,7 @@ static void fill_balloon(struct virtio_b
>  		vb->num_pages++;
>  		list_add(&page->lru, &vb->pages);
>  	}
> +	oom_killer_enable();
>  
>  	/* Didn't get any?  Oh well. */
>  	if (vb->num_pfns == 0)
> --
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