linux-kernel - Re: [PATCH -mm -v2] mm, swap, frontswap: Fix THP swap if frontswap enabled

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Date:   Thu, 8 Feb 2018 20:22:51 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Huang, Ying" <ying.huang@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Huang Ying <huang.ying.caritas@...il.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Dan Streetman <ddstreet@...e.org>,
        Seth Jennings <sjenning@...hat.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Shaohua Li <shli@...nel.org>, Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Shakeel Butt <shakeelb@...gle.com>, stable@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH -mm -v2] mm, swap, frontswap: Fix THP swap if frontswap
 enabled

On (02/08/18 02:25), Minchan Kim wrote:
[..]
> > >  	if (PageTransHuge(page)) {
> > > -		if (IS_ENABLED(CONFIG_THP_SWAP))
> > > -			get_swap_pages(1, true, &entry);
> > > +		/* Frontswap doesn't support THP */
> > > +		if (!frontswap_enabled()) {
> > > +			if (IS_ENABLED(CONFIG_THP_SWAP))
> > > +				get_swap_pages(1, true, &entry);
> > > +		}
> > >  		return entry;
> > >  	}
> > 
> > I have proposed exactly the same thing [1], Minchan commented that
> > it would introduce frontswap dependency to swap_slots.c [2]. Which
> > is true, but I'd still probably prefer to handle it all in
> > get_swap_page. Minchan, any objections?
> 
> I didn't want to spread out frontswap stuff unless it has good value
> because most of frontswap functions are located in mm/swapfile.c
> at this moment.

Sure, your points are perfectly valid. At the same time it might be the
case that we already kind of expose that THP dependency thing to vmscan.
The whole

	if (!add_to_swap()) {
		if (!PageTransHuge(page))
			goto activate_locked;

		split_huge_page_to_list(page);
		add_to_swap(page);
	}

looks a bit suspicious - if add_to_swap() fails and the page is THP then
split it and add_to_swap() again.

	-ss