linux-kernel - Re: [PATCH v4 2/7] deactivate invalidated pages

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Date:	Wed, 8 Dec 2010 09:56:42 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Nick Piggin <npiggin@...nel.dk>, Mel Gorman <mel@....ul.ie>
Subject: Re: [PATCH v4 2/7] deactivate invalidated pages

On Wed, 8 Dec 2010 07:51:25 +0900
Minchan Kim <minchan.kim@...il.com> wrote:

> On Wed, Dec 8, 2010 at 12:56 AM, Johannes Weiner <hannes@...xchg.org> wrote:
> > On Wed, Dec 08, 2010 at 12:26:25AM +0900, Minchan Kim wrote:
> >> On Tue, Dec 07, 2010 at 04:19:39PM +0100, Johannes Weiner wrote:
> >> > On Wed, Dec 08, 2010 at 12:07:10AM +0900, Minchan Kim wrote:
> >> > > On Tue, Dec 07, 2010 at 03:49:24PM +0100, Johannes Weiner wrote:
> >> > > > On Mon, Dec 06, 2010 at 02:29:10AM +0900, Minchan Kim wrote:
> >> > > > > Changelog since v3:
> >> > > > >  - Change function comments - suggested by Johannes
> >> > > > >  - Change function name - suggested by Johannes
> >> > > > >  - add only dirty/writeback pages to deactive pagevec
> >> > > >
> >> > > > Why the extra check?
> >> > > >
> >> > > > > @@ -359,8 +360,16 @@ unsigned long invalidate_mapping_pages(struct address_space *mapping,
> >> > > > >                       if (lock_failed)
> >> > > > >                               continue;
> >> > > > >
> >> > > > > -                     ret += invalidate_inode_page(page);
> >> > > > > -
> >> > > > > +                     ret = invalidate_inode_page(page);
> >> > > > > +                     /*
> >> > > > > +                      * If the page is dirty or under writeback, we can not
> >> > > > > +                      * invalidate it now.  But we assume that attempted
> >> > > > > +                      * invalidation is a hint that the page is no longer
> >> > > > > +                      * of interest and try to speed up its reclaim.
> >> > > > > +                      */
> >> > > > > +                     if (!ret && (PageDirty(page) || PageWriteback(page)))
> >> > > > > +                             deactivate_page(page);
> >> > > >
> >> > > > The writeback completion handler does not take the page lock, so you
> >> > > > can still miss pages that finish writeback before this test, no?
> >> > >
> >> > > Yes. but I think it's rare and even though it happens, it's not critical.
> >> > > >
> >> > > > Can you explain why you felt the need to add these checks?
> >> > >
> >> > > invalidate_inode_page can return 0 although the pages is !{dirty|writeback}.
> >> > > Look invalidate_complete_page. As easiest example, if the page has buffer and
> >> > > try_to_release_page can't release the buffer, it could return 0.
> >> >
> >> > Ok, but somebody still tried to truncate the page, so why shouldn't we
> >> > try to reclaim it?  The reason for deactivating at this location is
> >> > that truncation is a strong hint for reclaim, not that it failed due
> >> > to dirty/writeback pages.
> >> >
> >> > What's the problem with deactivating pages where try_to_release_page()
> >> > failed?
> >>
> >> If try_to_release_page fails and the such pages stay long time in pagevec,
> >> pagevec drain often happens.
> >
> > You mean because the pagevec becomes full more often?  These are not
> > many pages you get extra without the checks, the race window is very
> > small after all.
> 
> Right.
> It was a totally bad answer. The work in midnight makes my mind to be hurt. :)
> 
> Another point is that we can move such pages(!try_to_release_page,
> someone else holding the ref) into tail of inactive.
> We can't expect such pages will be freed sooner or later and it can
> stir lru pages unnecessary.
> On the other hand it's a _really_ rare so couldn't we move the pages into tail?
> If it can be justified, I will remove the check.
> What do you think about it?
> 

I wonder ...how about adding "victim" list for "Reclaim" pages ? Then, we don't need
extra LRU rotation.

Thanks,
-Kame


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