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

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Message-ID: <20180208013635.GA596@jagdpanzerIV>
Date:   Thu, 8 Feb 2018 10:36:35 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>
Cc:     "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 (02/07/18 13:05), Andrew Morton wrote:
[..]
> hm.  This is assuming that "cluster==true" means "this is thp swap". 
> That's presently true, but is it appropriate that get_swap_pages() is
> peeking at "cluster" to work out why it is being called?
> 
> Or would it be cleaner to do this in get_swap_page()?  Something like
> 
> --- a/mm/swap_slots.c~a
> +++ a/mm/swap_slots.c
> @@ -317,8 +317,11 @@ swp_entry_t get_swap_page(struct page *p
>  	entry.val = 0;
>  
>  	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?

[1] https://marc.info/?l=linux-mm&m=151791052007719&w=2
[2] https://marc.info/?l=linux-mm&m=151792646812617&w=2

	-ss