linux-kernel - Re: [PATCH 06/10] ksm: identify PageKsm pages

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Message-ID: <Pine.LNX.4.64.0907221346040.529@sister.anvils>
Date:	Wed, 22 Jul 2009 13:54:06 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Rik van Riel <riel@...hat.com>
cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Izik Eidus <ieidus@...hat.com>, akpm@...ux-foundation.org,
	chrisw@...hat.com, avi@...hat.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, nickpiggin@...oo.com.au,
	Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [PATCH 06/10] ksm: identify PageKsm pages

On Tue, 21 Jul 2009, Rik van Riel wrote:
> Andrea Arcangeli wrote:
> > > -	if (PageAnon(old_page)) {
> > > +	if (PageAnon(old_page) && !PageKsm(old_page)) {
> > >    if (!trylock_page(old_page)) {
> > >     page_cache_get(old_page);
> > >     pte_unmap_unlock(page_table, ptl);
> > 
> > What exactly does it buy to have PageAnon return 1 on ksm pages,
> > besides requiring the above additional check (that if we stick to the
> > above code, I would find safer to move inside reuse_swap_page).
> 
> I guess that if they are to remain unswappable, they
> should go onto the unevictable list.

The KSM pages are not put on any LRU, so wouldn't be slowing vmscan
down with futile scans: isn't the unevictable list for pages which
belong to another LRU once they become evictable again?

(At this instant I've forgotten why there's an unevictable list at
all - somewhere in vmscan.c which is accustomed to dealing with
pages on lists, so easier to have them on a list than not?)

> 
> Then again, I'm guessing this is all about to change
> in not too much time :)

Yes, I'd much rather put the effort into making them swappable,
than fiddling with counts here and there to highlight their
current unswappability.

Hugh
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