linux-kernel - Re: [PATCH v2] fix mlocked page counter mistmatch

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Date:	Wed, 4 Feb 2009 08:44:08 +0900
From:	MinChan Kim <minchan.kim@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux mm <linux-mm@...ck.org>,
	linux kernel <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH v2] fix mlocked page counter mistmatch

On Wed, Feb 04, 2009 at 01:44:52AM +0900, KOSAKI Motohiro wrote:
> Hi MinChan,
> 
> I'm confusing now.
> Can you teach me?

No problem. :)

> 
> > When I tested following program, I found that mlocked counter
> > is strange.
> > It couldn't free some mlocked pages of test program.
> > It is caused that try_to_unmap_file don't check real
> > page mapping in vmas.
> 
> What meanining is "real" page mapping?

What I mean is that if the page is mapped at the vma,
I call it's "real" page mapping.
I explain it more detaily below.

> 
> 
> > That's because goal of address_space for file is to find all processes
> > into which the file's specific interval is mapped.
> > What I mean is that it's not related page but file's interval.
> 
> hmmm. No.
> I ran your reproduce program.
> 
> two vma pointing the same page cause this leaking.

I don't think so. 

> 
> iow, any library have .text and .data segment. then the tail of .text
> and the head of .data vma point the same page.
> its page was leaked.
> 
> 
> > Even if the page isn't really mapping at the vma, it returns
> > SWAP_MLOCK since the vma have VM_LOCKED, then calls
> > try_to_mlock_page. After all, mlocked counter is increased again.
> >
> > COWed anon page in a file-backed vma could be a such case.
> > This patch resolves it.
> 
> What meaning is "anon page in a file-backed"?
> As far as I know, if cow happend on private mapping page, new page is
> treated truth anon.
> 

vm_area_struct's annotation can explain about your question. 

struct vm_area_struct {
  struct mm_struct * vm_mm; /* The address space we belong to. */
  ....
  ....
  /*  
   * A file's MAP_PRIVATE vma can be in both i_mmap tree and anon_vma
   * list, after a COW of one of the file pages.  A MAP_SHARED vma
   * can only be in the i_mmap tree.  An anonymous MAP_PRIVATE, stack
   * or brk vma (with NULL file) can only be in an anon_vma list.
   */
  struct list_head anon_vma_node; /* Serialized by anon_vma->lock */
  struct anon_vma *anon_vma;  /* Serialized by page_table_lock */
  ....
  ....
}

Let us call it anon page in a file-backed. 
In this case, the new page is mapped at the vma. 
the vma don't include old page any more but i_mmap tree still have 
the vma. 

So, the i_mmap tree can have the vma which don't include
the page if the one is anon page in a file-backed. 

This problem is caused by that. 
Is it enough ?

> 
> So, I don't reach to your conclusion yet. please teach me.

-- 
Kinds Regards
MinChan Kim

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