linux-ext4 - Re: page eviction from the buddy cache

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Message-Id: <20130423150008.046ee9351da4681128db0bf3@linux-foundation.org>
Date:	Tue, 23 Apr 2013 15:00:08 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	Bernd Schubert <bernd.schubert@...tmail.fm>,
	Alexey Lyahkov <alexey.lyashkov@...il.com>,
	Will Huck <will.huckk@...il.com>,
	Andrew Perepechko <anserper@...ru>, linux-ext4@...r.kernel.org,
	linux-mm@...ck.org, mgorman@...e.de
Subject: Re: page eviction from the buddy cache

On Tue, 23 Apr 2013 12:57:45 -0700 (PDT) Hugh Dickins <hughd@...gle.com> wrote:

> For now I stand by what I said before (if you find it effective
> in practice - I haven't heard back): at the moment you need to
> 
> 	mark_page_accessed(page);	/* to SetPageReferenced */
> 	lru_add_drain();		/* to SetPageLRU */
> 	mark_page_accessed(page);	/* to SetPageActive */
> 
> when such a metadata page is first brought in.

That should fix things for now.  Although it might be better to just do

 	mark_page_accessed(page);	/* to SetPageReferenced */
 	lru_add_drain();		/* to SetPageLRU */

Because a) this was too early to decide that the page is
super-important and b) the second touch of this page should have a
mark_page_accessed() in it already.

I do agree that we should be able to set both PageReferenced and
PageActive on a lru_add_pvecs page and have those hints honoured when
lru_add_pvecs is spilled onto the LRU.

At present the code decides up-front which LRU the lru_add_pvecs page
will eventually be spilled onto.  That's a bit strange and I wonder why
we did it that way.  Why not just have a single (per-cpu) magazine of
pages which are to go onto the LRUs, and decide *which* LRU that will
be at the last possible moment?

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