Re: [PATCH v2] fix mlocked page counter mistmatch

MinChan Kim
2009-02-03T23:44:34Z

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