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

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Message-ID: <alpine.LNX.2.00.1304231518430.14137@eggly.anvils>
Date:	Tue, 23 Apr 2013 15:31:02 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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, Andrew Morton wrote:
> On Tue, 23 Apr 2013 12:57:45 -0700 (PDT) Hugh Dickins <hughd@...gle.com> wrote:
> 
> 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?

Yes, it is strange, and I'm wanting to get away from that: though I
won't be surprised if we discover that it's actually important for
avoiding races in the current scheme - which won't cope well with a
page being marked PageActive at the wrong moment (an instant after
it has been placed on the Inactive list).

What I want is for pages on the the per-cpu lru_add_pvecs to be already
marked PageLRU, and without raised page count, so that they are eligible
for isolation and migration without needing a drain.  (But I may be too
ambitious in trying to avoid the raised page count for lru_rotate_pvecs
and the others too.)

Hugh
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