linux-kernel - Re: [PATCH] vmscan: memcg needs may_swap (Re: [patch] vmscan: rename sc.may_swap to may

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Message-Id: <20090401140828.755f87aa.nishimura@mxp.nes.nec.co.jp>
Date:	Wed, 1 Apr 2009 14:08:28 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	nishimura@....nes.nec.co.jp,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Minchan Kim <minchan.kim@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Rik van Riel <riel@...hat.com>,
	Balbir Singh <balbir@...ibm.com>
Subject: Re: [PATCH] vmscan: memcg needs may_swap (Re: [patch] vmscan:
 rename  sc.may_swap to may_unmap)

On Wed, 1 Apr 2009 06:09:51 +0200, Johannes Weiner <hannes@...xchg.org> wrote:
> On Tue, Mar 31, 2009 at 10:48:32AM +0900, KOSAKI Motohiro wrote:
> > > > Sorry for too late response.
> > > > I don't know memcg well.
> > > > 
> > > > The memcg managed to use may_swap well with global page reclaim until now.
> > > > I think that was because may_swap can represent both meaning.
> > > > Do we need each variables really ?
> > > > 
> > > > How about using union variable ?
> > > 
> > > or Just removing one of them  ?
> > 
> > I hope all may_unmap user convert to using may_swap.
> > may_swap is more efficient and cleaner meaning.
> 
> How about making may_swap mean the following:
> 
> 	@@ -642,6 +639,8 @@ static unsigned long shrink_page_list(st
> 	 		 * Try to allocate it some swap space here.
> 	 		 */
> 	 		if (PageAnon(page) && !PageSwapCache(page)) {
> 	+			if (!sc->map_swap)
> 	+				goto keep_locked;
> 	 			if (!(sc->gfp_mask & __GFP_IO))
> 	 				goto keep_locked;
> 	 			if (!add_to_swap(page))
> 
but it doesn't work for shmem/tmpfs, does it?
So, I did in my first patch like:

@@ -616,6 +619,11 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		if (!sc->may_unmap && page_mapped(page))
 			goto keep_locked;
 
+		if (!sc->may_swap && PageSwapBacked(page)
+			/* SwapCache uses 'swap' already */
+			&& !PageSwapCache(page))
+			goto keep_locked;
+
 		/* Double the slab pressure for mapped and swapcache pages */
 		if (page_mapped(page) || PageSwapCache(page))
 			sc->nr_scanned++;

> try_to_free_pages() always sets it.
> 
> try_to_free_mem_cgroup_pages() sets it depending on whether it really
> wants swapping, and only swapping, right?
right.

> But the above would still reclaim already swapped anon pages
then, it would be better to add a check at shrink_page_list anyway..

Kosaki-san, what do you think?


Thanks,
Daisuke Nishimura.

> and I don't know the memory
> controller.
> 
> balance_pgdat() always sets it.
> 
> __zone_reclaim() sets it depending on zone_reclaim_mode.  The
> RECLAIM_SWAP bit of this field and its documentation in
> Documentation/sysctl/vm.txt suggests it also really only means swap.
> 
> shrink_all_memory() would be the sole user of may_unmap because it
> really wants to eat cache first.  But this could be figured out on a
> different occasion.
> 
> 	Hannes
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