linux-kernel - Re: [RFC 1/2] deactive invalidated pages

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Date:	Mon, 22 Nov 2010 21:48:14 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Nick Piggin <npiggin@...nel.dk>, Mel Gorman <mel@....ul.ie>
Subject: Re: [RFC 1/2] deactive invalidated pages

On Tue, 23 Nov 2010 14:45:15 +0900 Minchan Kim <minchan.kim@...il.com> wrote:

> On Tue, Nov 23, 2010 at 2:22 PM, Andrew Morton
> <akpm@...ux-foundation.org> wrote:
> > On Tue, 23 Nov 2010 14:23:33 +0900 Minchan Kim <minchan.kim@...il.com> wrote:
> >
> >> On Tue, Nov 23, 2010 at 2:01 PM, Andrew Morton
> >> <akpm@...ux-foundation.org> wrote:
> >> > On Tue, 23 Nov 2010 13:52:05 +0900 Minchan Kim <minchan.kim@...il.com> wrote:
> >> >
> >> >> >> +/*
> >> >> >> + * Function used to forecefully demote a page to the head of the inactive
> >> >> >> + * list.
> >> >> >> + */
> >> >> >
> >> >> > This comment is wrong? __The page gets moved to the _tail_ of the
> >> >> > inactive list?
> >> >>
> >> >> No. I add it in _head_ of the inactive list intentionally.
> >> >> Why I don't add it to _tail_ is that I don't want to be aggressive.
> >> >> The page might be real working set. So I want to give a chance to
> >> >> activate it again.
> >> >
> >> > Well.. __why? __The user just tried to toss the page away altogether. __If
> >> > the kernel wasn't able to do that immediately, the best it can do is to
> >> > toss the page away asap?
> >> >
> >> >> If it's not working set, it can be reclaimed easily and it can prevent
> >> >> active page demotion since inactive list size would be big enough for
> >> >> not calling shrink_active_list.
> >> >
> >> > What is "working set"? __Mapped and unmapped pagecache, or are you
> >> > referring solely to mapped pagecache?
> >>
> >> I mean it's mapped by other processes.
> >>
> >> >
> >> > If it's mapped pagecache then the user was being a bit silly (or didn't
> >> > know that some other process had mapped the file). __In which case we
> >> > need to decide what to do - leave the page alone, deactivate it, or
> >> > half-deactivate it as this patch does.
> >>
> >>
> >> What I want is the half-deactivate.
> >>
> >> Okay. We will use the result of invalidate_inode_page.
> >> If fail happens by page_mapped, we can do half-deactivate.
> >> But if fail happens by dirty(ex, writeback), we can add it to tail.
> >> Does it make sense?
> >
> > Spose so. __It's unobvious.
> >
> > If the page is dirty or under writeback then reclaim will immediately
> > move it to the head of the LRU anyway. __But given that the user has
> 
> Why does it move into head of LRU?
> If the page which isn't mapped doesn't have PG_referenced, it would be
> reclaimed.

If it's dirty or under writeback it can't be reclaimed!

> > just freed a bunch of pages with invalidate(), it's unlikely that
> > reclaim will be running soon.
> 
> If reclaim doesn't start soon, it's good. That's because we have a
> time to activate it and
> when reclaim happens, reclaimer can reclaim pages easily.
> 
> If I don't understand your point, could you elaborate on it?

If reclaim doesn't happen soon and the page was dirty or under
writeback (and hence unreclaimable) then there's a better chance that
it _will_ be reclaimable by the time reclaim comes along and has a look
at it.  Yes, that's good.

And a note to Mel: this is one way in which we can get significant
(perhaps tremendous) numbers of dirty pages coming off the tail of the
LRU, and hence eligible for pageout() treatment.
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