linux-kernel - Re: [patch] mm: release swap slots for actively used pages

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Message-Id: <20090528092345.58f31056.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 28 May 2009 09:23:45 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, riel@...hat.com,
	hugh.dickins@...cali.co.uk
Subject: Re: [patch] mm: release swap slots for actively used pages

On Wed, 27 May 2009 16:15:35 -0700
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Wed, 27 May 2009 03:47:39 +0200
> Johannes Weiner <hannes@...xchg.org> wrote:
> 
> > For anonymous pages activated by the reclaim scan or faulted from an
> > evicted page table entry we should always try to free up swap space.
> > 
> > Both events indicate that the page is in active use and a possible
> > change in the working set.  Thus removing the slot association from
> > the page increases the chance of the page being placed near its new
> > LRU buddies on the next eviction and helps keeping the amount of stale
> > swap cache entries low.
> > 
> > try_to_free_swap() inherently only succeeds when the last user of the
> > swap slot vanishes so it is safe to use from places where that single
> > mapping just brought the page back to life.
> > 
> 
> Seems that this has a risk of worsening swap fragmentation for some
> situations.  Or not, I have no way of knowing, really.
> 
I'm afraid, too.

> > diff --git a/mm/memory.c b/mm/memory.c
> > index 8b4e40e..407ebf7 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -2671,8 +2671,7 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
> >  	mem_cgroup_commit_charge_swapin(page, ptr);
> >  
> >  	swap_free(entry);
> > -	if (vm_swap_full() || (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
> > -		try_to_free_swap(page);
> > +	try_to_free_swap(page);
> >  	unlock_page(page);
> >  
> >  	if (write_access) {
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 621708f..2f0549d 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -788,7 +788,7 @@ cull_mlocked:
> >  
> >  activate_locked:
> >  		/* Not a candidate for swapping, so reclaim swap space. */
> > -		if (PageSwapCache(page) && vm_swap_full())
> > +		if (PageSwapCache(page))
> >  			try_to_free_swap(page);
> >  		VM_BUG_ON(PageActive(page));
> >  		SetPageActive(page);
> 
> How are we to know that this is a desirable patch for Linux??

I'm not sure what is the "purpose/benefit" of this patch...
In patch description,
"we should always try to free up swap space" ...then, why "should" ?

Thanks,
-Kame

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