linux-kernel - Re: [PATCH RESEND] mm: don't zero ballooned pages

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Date:   Thu, 3 Aug 2017 15:24:01 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        virtualization@...ts.linux-foundation.org, mhocko@...nel.org,
        zhenwei.pi@...runcloud.com, akpm@...ux-foundation.org,
        dave.hansen@...el.com, mawilcox@...rosoft.com
Subject: Re: [PATCH RESEND] mm: don't zero ballooned pages

On Thu, Aug 03, 2017 at 07:59:17PM +0800, Wei Wang wrote:
> This patch is a revert of 'commit bb01b64cfab7 ("mm/balloon_compaction.c:
> enqueue zero page to balloon device")'
> 
> Ballooned pages will be marked as MADV_DONTNEED by the hypervisor and
> shouldn't be given to the host ksmd to scan. Therefore, it is not
> necessary to zero ballooned pages, which is very time consuming when
> the page amount is large. The ongoing fast balloon tests show that the
> time to balloon 7G pages is increased from ~491ms to 2.8 seconds with
> __GFP_ZERO added. So, this patch removes the flag.
> 
> Signed-off-by: Wei Wang <wei.w.wang@...el.com>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Michael S. Tsirkin <mst@...hat.com>

Fixes: bb01b64cfab7 ("mm/balloon_compaction.c: enqueue zero page to balloon device")

Looks like hypervisor is better placed to zero these if it wants to.
If it can't for some reason, this change would need a feature bit
to avoid adding extra work for all guests.

Acked-by: Michael S. Tsirkin <mst@...hat.com>

> ---
>  mm/balloon_compaction.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
> index 9075aa5..b06d9fe 100644
> --- a/mm/balloon_compaction.c
> +++ b/mm/balloon_compaction.c
> @@ -24,7 +24,7 @@ struct page *balloon_page_enqueue(struct balloon_dev_info *b_dev_info)
>  {
>  	unsigned long flags;
>  	struct page *page = alloc_page(balloon_mapping_gfp_mask() |
> -				__GFP_NOMEMALLOC | __GFP_NORETRY | __GFP_ZERO);
> +				       __GFP_NOMEMALLOC | __GFP_NORETRY);
>  	if (!page)
>  		return NULL;
>  
> -- 
> 2.7.4