Re: [PATCH v2] fix mlocked page counter mistmatch

[MinChan Kim <minchan.kim@gmail.com>]

On Wed, Feb 04, 2009 at 11:12:37AM +0900, KOSAKI Motohiro wrote:
> > 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. 
> 
> Please confirm by actual machine and kernel.
> I confirmed by printk debugging.
> 



It seems that we have a misundersting.
I think you can't understand my point. Sorry for my poor english.
You're right and i also already tested it, of course.
two vmas point to same address but have a different page due to COW.
So, What I mean is that problem is lack of page_check_address.
It causes this problem. :)

> 
> > > 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. 
> 
> hmhm. thanks. 
> my understanding largely improvement.
> 
> I agree page_check_address() checking is necessary.
> 
> > 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 ?
> 
> Could you please teach me why this issue doesn't happend on munlockall()?
> your scenario seems to don't depend on exit_mmap().


Good question.
It's a different issue.
It is related to mmap_sem locking issue. 

Actually, I am about to make a patch.
But, I can't understand that Why try_do_mlock_page should downgrade mm_sem ?
Is it necessary ? 

In munlockall path, mmap_sem already is holding in write-mode of mmap_sem.
so, try_to_mlock_page always fail to downgrade mmap_sem.
It's why it looks like working well about mlocked counter. 


> 
> 

-- 
Kinds Regards
MinChan Kim

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