linux-kernel - Re: [PATCH -mm 17/25] Mlocked Pages are non-reclaimable

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Date:	Tue, 10 Jun 2008 17:43:17 -0400
From:	Lee Schermerhorn <Lee.Schermerhorn@...com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, kosaki.motohiro@...fujitsu.com,
	linux-mm@...ck.org, eric.whitney@...com
Subject: Re: [PATCH -mm 17/25] Mlocked Pages are non-reclaimable

On Tue, 2008-06-10 at 17:14 -0400, Rik van Riel wrote:
> On Tue, 10 Jun 2008 05:31:30 +0200
> Nick Piggin <npiggin@...e.de> wrote:
> 
> > If we eventually run out of page flags on 32 bit, then sure this might be
> > one we could look at geting rid of. Once the code has proven itself.
> 
> Yes, after the code has proven stable, we can probably get
> rid of the PG_mlocked bit and use only PG_unevictable to mark
> these pages.
> 
> Lee, Kosaki-san, do you see any problem with that approach?
> Is the PG_mlocked bit really necessary for non-debugging
> purposes?
> 

Well, it does speed up the check for mlocked pages in page_reclaimable()
[now page_evictable()?] as we don't have to walk the reverse map to
determine that a page is mlocked.   In many places where we currently
test page_reclaimable(), we really don't want to and maybe can't walk
the reverse map.

Unless you're evisioning even larger rework, the PG_unevictable flag
[formerly PG_noreclaim, right?] is analogous to PG_active.  It's only
set when the page is on the corresponding lru list or being held
isolated from it, temporarily.  See isolate_lru_page() and
putback_lru_page() and users thereof--such as mlock_vma_page().  Again,
I have seen what changes you're making here, so maybe that's all
changing.  But, currently, PG_unevictable would not be a replacement for
PG_mlocked.

Anyway, let's see what you come up with before we tackle this.

Couple of related items:

+ 26-rc5-mm1 + a small fix to the double unlock_page() in
shrink_page_list() has been running for a couple of hours on my 32G,
16cpu ia64 numa platform w/o error.  Seems to have survived the merge
into -mm, despite the issues Andrew has raised.

+ on same platform, Mel Gorman's mminit debug code is reporting that
we're using 22 page flags with Noreclaim, Mlock and PAGEFLAGS_EXTENDED
configured.

Lee

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