From mboxrd@z Thu Jan  1 00:00:00 1970
From: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
Subject: Re: [PATCH -mm -v10 1/3] mm, THP, swap: Delay splitting THP during
 swap out
Date: Thu, 27 Apr 2017 09:37:09 -0400
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To: "Huang, Ying" <ying.huang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
Cc: Minchan Kim <minchan-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Andrea Arcangeli <aarcange-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Ebru Akagunduz <ebru.akagunduz-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Hugh Dickins <hughd-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Shaohua Li <shli-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Rik van Riel <riel-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org

On Thu, Apr 27, 2017 at 03:12:34PM +0800, Huang, Ying wrote:
> Minchan Kim <minchan-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> writes:
> > On Tue, Apr 25, 2017 at 08:56:56PM +0800, Huang, Ying wrote:
> >> @@ -178,20 +192,12 @@ int add_to_swap(struct page *page, struct list_head *list)
> >>  	VM_BUG_ON_PAGE(!PageLocked(page), page);
> >>  	VM_BUG_ON_PAGE(!PageUptodate(page), page);
> >>  
> >> -	entry = get_swap_page();
> >> +retry:
> >> +	entry = get_swap_page(page);
> >>  	if (!entry.val)
> >> -		return 0;
> >> -
> >> -	if (mem_cgroup_try_charge_swap(page, entry)) {
> >> -		swapcache_free(entry);
> >> -		return 0;
> >> -	}
> >> -
> >> -	if (unlikely(PageTransHuge(page)))
> >> -		if (unlikely(split_huge_page_to_list(page, list))) {
> >> -			swapcache_free(entry);
> >> -			return 0;
> >> -		}
> >> +		goto fail;
> >
> > So, with non-SSD swap, THP page *always* get the fail to get swp_entry_t
> > and retry after split the page. However, it makes unncessary get_swap_pages
> > call which is not trivial. If there is no SSD swap, thp-swap out should
> > be void without adding any performance overhead.
> > Hmm, but I have no good idea to do it simple. :(
> 
> For HDD swap, the device raw throughput is so low (< 100M Bps
> typically), that the added overhead here will not be a big issue.  Do
> you agree?

I fully agree. If you swap to spinning rust, an extra function call
here is the least of your concern.