linux-kernel - Re: [PATCH 2/2 V2] ksm: take dirty bit as reference to avoid volatile pages scanning

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Date:	Wed, 22 Jun 2011 08:04:12 +0800
From:	Nai Xia <nai.xia@...il.com>
To:	Chris Wright <chrisw@...s-sol.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Izik Eidus <izik.eidus@...ellosystems.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Chris Wright <chrisw@...s-sol.org>,
	Rik van Riel <riel@...hat.com>,
	"linux-mm" <linux-mm@...ck.org>,
	Johannes Weiner <hannes@...xchg.org>,
	"linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2 V2] ksm: take dirty bit as reference to avoid volatile pages scanning

(Sorry for repeated mail, I forgot to Cc the list..)

On Wednesday 22 June 2011 06:38:00 you wrote:
> * Nai Xia (nai.xia@...il.com) wrote:
> > Introduced ksm_page_changed() to reference the dirty bit of a pte. We clear 
> > the dirty bit for each pte scanned but don't flush the tlb. For a huge page, 
> > if one of the subpage has changed, we try to skip the whole huge page 
> > assuming(this is true by now) that ksmd linearly scans the address space.
> 
> This doesn't build w/ kvm as a module.

I think it's because of the name-error of a related kvm patch, which I only sent
in a same email thread. http://marc.info/?l=linux-mm&m=130866318804277&w=2
The patch split is not clean...I'll redo it.

> 
> > A NEW_FLAG is also introduced as a status of rmap_item to make ksmd scan
> > more aggressively for new VMAs - only skip the pages considered to be volatile
> > by the dirty bits. This can be enabled/disabled through KSM's sysfs interface.
> 
> This seems like it should be separated out.  And while it might be useful
> to enable/disable for testing, I don't think it's worth supporting for
> the long term.  Would also be useful to see the value of this flag.

I think it maybe useful for uses who want to turn on/off this scan policy explicitly
according to their working sets? 

> 
> > @@ -454,7 +468,7 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node)
> >  		else
> >  			ksm_pages_shared--;
> >  		put_anon_vma(rmap_item->anon_vma);
> > -		rmap_item->address &= PAGE_MASK;
> > +		rmap_item->address &= ~STABLE_FLAG;
> >  		cond_resched();
> >  	}
> >  
> > @@ -542,7 +556,7 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
> >  			ksm_pages_shared--;
> >  
> >  		put_anon_vma(rmap_item->anon_vma);
> > -		rmap_item->address &= PAGE_MASK;
> > +		rmap_item->address &= ~STABLE_FLAG;
> >  
> >  	} else if (rmap_item->address & UNSTABLE_FLAG) {
> >  		unsigned char age;
> > @@ -554,12 +568,14 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
> >  		 * than left over from before.
> >  		 */
> >  		age = (unsigned char)(ksm_scan.seqnr - rmap_item->address);
> > -		BUG_ON(age > 1);
> > +		BUG_ON (age > 1);
> 
> No need to add space after BUG_ON() there
> 
> > +
> >  		if (!age)
> >  			rb_erase(&rmap_item->node, &root_unstable_tree);
> >  
> >  		ksm_pages_unshared--;
> > -		rmap_item->address &= PAGE_MASK;
> > +		rmap_item->address &= ~UNSTABLE_FLAG;
> > +		rmap_item->address &= ~SEQNR_MASK;
> 
> None of these changes are needed AFAICT.  &= PAGE_MASK clears all
> relevant bits.  How could it be in a tree, have NEW_FLAG set, and
> while removing from tree want to preserve NEW_FLAG?

You are right, it's meaningless to preserve NEW_FLAG after it goes 
through the trees. I'll revert the lines.

Thanks!

Nai

> 
> thanks,
> -chris
> 
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