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

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Date:	Tue, 23 Apr 2013 12:57:45 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Theodore Ts'o <tytso@....edu>
cc:	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,
	akpm@...ux-foundation.org, linux-mm@...ck.org, mgorman@...e.de
Subject: Re: page eviction from the buddy cache

On Tue, 23 Apr 2013, Theodore Ts'o wrote:
> On Tue, Apr 23, 2013 at 02:02:37PM +0200, Bernd Schubert wrote:
> > 
> > I just thought we can (mis)use that flag and and add another
> > information to the page that it holds meta data. The mm system then
> > could use that flag and evict those pages with a lower priority
> > compared to other pages.
> 
> Well, the flag I added was to the buffer_head, not to the page, and my
> understanding is that the mm folks are very hesitant to add new page
> flags, since they are bumping up against the 32 bit limit (on the i386
> platform), and they are trying to keep the struct page structure trim
> and svelte.  :-)

Yes indeed.  But luckily this issue seems not to need another page flag.

If metadata is useful, it gets used: so mark_page_accessed when it's used
should generally do the job; perhaps it's already called on that path,
perhaps calls need to be added.

Rarely used but nonetheless useful metadata might get pushed out by data.
But I don't think we want another page flag, and yet more complicated
reclaim policy in mm for that case.  If the filesystem knows of such
cases, I hope it can find a way to use mark_page_accessed more often
on such pages than they are actually accessed, to help retain them.

What this thread did bring up was the failure of mark_page_accessed
to be fully effective until the page is flushed from pagevec to lru.
That remains a good point: something that several would like to fix.

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.

We all hate that lru_add_drain in the middle, which will exacerbate
lru_lock contention.  We would love to eliminate the need for most
lru_add_drains: I have some ideas which I'm pursuing in odd moments,
but I promise nothing.

Hugh
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