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

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Date:	Wed, 24 Apr 2013 10:26:50 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Hugh Dickins <hughd@...gle.com>,
	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, Apr 23, 2013 at 03:00:08PM -0700, Andrew Morton wrote:
> 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.

The question is do we really want to put lru_add_drain() into the ext4
file system code?  That seems to pushing some fairly mm-specific
knowledge into file system code.  I'll do this if I have to do, but
wouldn't be better if this was pushed into mark_page_accessed(), or
some other new API was exported by the mm subsystem?

> 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?

And this is why it seems strange that fs code should need or should
want to put something as mm-implementation dependent into their code
paths.  At minimum, if we do this, we'll want to put some explanatory
comments so that later, people won't be asking, what the !@#@?!? are
the ext4 people calling lru_add_drain() here?

							- Ted
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