Re: [PATCH v2] fix mlocked page counter mistmatch

MinChan Kim
2009-02-04T02:45:27Z

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