linux-kernel - Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

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Date:	Fri, 26 Mar 2010 00:29:01 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped
 anonymous pages

Hi, Kame. 

On Thu, 2010-03-25 at 19:12 +0900, KAMEZAWA Hiroyuki wrote:
> On Thu, 25 Mar 2010 18:59:25 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
> 
> > > > > > Kosaki-san,
> > > > > > 
> > > > > >  IIUC, the race in memory-hotunplug was fixed by this patch [2/11].
> > > > > > 
> > > > > >  But, this behavior of unmap_and_move() requires access to _freed_
> > > > > >  objects (spinlock). Even if it's safe because of SLAB_DESTROY_BY_RCU,
> > > > > >  it't not good habit in general.
> > > > > > 
> > > > > >  After direct compaction, page-migration will be one of "core" code of
> > > > > >  memory management. Then, I agree to patch [1/11] as our direction for
> > > > > >  keeping sanity and showing direction to more updates. Maybe adding
> > > > > >  refcnt and removing RCU in futuer is good.
> > > > > 
> > > > > But Christoph seems oppose to remove SLAB_DESTROY_BY_RCU. then refcount
> > > > > is meaningless now.
> > > > 
> > > > Christoph is opposed to removing it because of cache-hotness issues more
> > > > so than use-after-free concerns. The refcount is needed with or without
> > > > SLAB_DESTROY_BY_RCU.
> > > > 
> > > 
> > > I wonder a code which the easiest to be read will be like following.
> > > ==
> > > 
> > >         if (PageAnon(page)) {
> > >                 struct anon_vma anon = page_lock_anon_vma(page);
> > > 		/* to take this lock, this page must be mapped. */
> > > 		if (!anon_vma)
> > > 			goto uncharge;
> > > 		increase refcnt
> > > 		page_unlock_anon_vma(anon);
> > >         }
> > > 	....
> > > ==
> > 
> > This seems very good and acceptable to me. This refcnt usage
> > obviously reduce rcu-lock holding time.
> > 
> > I still think no refcount doesn't cause any disaster. but I agree
> > this is forward step patch.
> > 
> 
> BTW, by above change and the change in patch [2/11], 
> "A page turnd to be SwapCache and free unmapped but not freed"
> page will be never migrated.
> 
> Mel, could you change the check as this ??
> 
> 	if (PageAnon(page)) {
> 		rcu_read_lock();
> 		if (!page_mapcount(page)) {
> 			rcu_read_unlock();
> 			if (!PageSwapCache(page))
> 				goto uncharge;
> 			/* unmapped swap cache can be migrated */


Which case do we have PageAnon && (page_mapcount == 0) && PageSwapCache ?
With looking over code which add_to_swap_cache, I found somewhere. 

1) shrink_page_list
I think this case doesn't matter by isolate_lru_xxx.

2) shmem_swapin
It seems to be !PageAnon

3) shmem_writepage
It seems to be !PageAnon. 

4) do_swap_page
page_add_anon_rmap increases _mapcount before setting page->mapping to anon_vma. 
So It doesn't matter. 


I think following codes in unmap_and_move seems to handle 3) case. 

---
         * Corner case handling:
         * 1. When a new swap-cache page is read into, it is added to the LRU
         * and treated as swapcache but it has no rmap yet.
        ...
        if (!page->mapping) {
                if (!PageAnon(page) && page_has_private(page)) {
                ....
                }    
                goto skip_unmap;
        }    

---

Do we really check PageSwapCache in there?
Do I miss any case?



-- 
Kind regards,
Minchan Kim


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