Re: [PATCH] zsmalloc: zs_page_migrate: schedule free_work if zspage is ZS_EMPTY

[Minchan Kim <minchan@kernel.org>]

Hi Hui,

On Mon, Aug 14, 2017 at 02:34:46PM +0800, Hui Zhu wrote:
> After commit e2846124f9a2 ("zsmalloc: zs_page_migrate: skip unnecessary
> loops but not return -EBUSY if zspage is not inuse") zs_page_migrate
> can handle the ZS_EMPTY zspage.
> 
> But it will affect the free_work free the zspage.  That will make this
> ZS_EMPTY zspage stay in system until another zspage wake up free_work.
> 
> Make this patch let zs_page_migrate wake up free_work if need.
> 
> Fixes: e2846124f9a2 ("zsmalloc: zs_page_migrate: skip unnecessary loops but not return -EBUSY if zspage is not inuse")
> Signed-off-by: Hui Zhu <zhuhui@xiaomi.com>

This patch makes me remind why I didn't try to migrate empty zspage
as you did e2846124f9a2. I have forgotten it toally.

We cannot guarantee when the freeing of the page happens if we use
deferred freeing in zs_page_migrate. However, we returns
MIGRATEPAGE_SUCCESS which is totally lie.
Without instant freeing the page, it doesn't help the migration
situation. No?

I start to wonder why your patch e2846124f9a2 helped your test.
I will think over the issue with fresh mind after the holiday.

> ---
>  mm/zsmalloc.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 62457eb..48ce043 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -2035,8 +2035,14 @@ int zs_page_migrate(struct address_space *mapping, struct page *newpage,
>  	 * Page migration is done so let's putback isolated zspage to
>  	 * the list if @page is final isolated subpage in the zspage.
>  	 */
> -	if (!is_zspage_isolated(zspage))
> -		putback_zspage(class, zspage);
> +	if (!is_zspage_isolated(zspage)) {
> +		/*
> +		 * The page and class is locked, we cannot free zspage
> +		 * immediately so let's defer.
> +		 */
> +		if (putback_zspage(class, zspage) == ZS_EMPTY)
> +			schedule_work(&pool->free_work);
> +	}
>  
>  	reset_page(page);
>  	put_page(page);
> -- 
> 1.9.1
> 
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