Re: [PATCH v2] fix mlocked page counter mistmatch

KOSAKI Motohiro
2009-02-04T10:28:31Z

> With '29-rc3-git5', I found,
>
> static int try_to_mlock_page(struct page *page, struct vm_area_struct *vma)
> {
> int mlocked = 0;
>
> if (down_read_trylock(&vma->vm_mm->mmap_sem)) {
> if (vma->vm_flags & VM_LOCKED) {
> mlock_vma_page(page);
> mlocked++; /* really mlocked the page */
> }
> up_read(&vma->vm_mm->mmap_sem);
> }
> return mlocked;
> }
>
> It still try to downgrade mmap_sem.
> Do I miss something ?

sorry, I misunderstood your "downgrade". I said linus removed downgrade_write(&mma_sem).

Now, I understand this issue perfectly. I agree you and lee-san's fix is correct.
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

and, I think current try_to_mlock_page() is correct. no need change.
Why?

1. Generally, mmap_sem holding is necessary when vma->vm_flags accessed.
that's vma's basic rule.
2. However, try_to_unmap_one() doesn't held mamp_sem. but that's ok.
it often get incorrect result. but caller consider incorrect value safe.
3. try_to_mlock_page() need mmap_sem because it obey rule (1).
4. in try_to_mlock_page(), if down_read_trylock() is failure,
we can't move the page to unevictable list. but that's ok.
the page in evictable list is periodically try to reclaim. and
be called try_to_unmap().
try_to_unmap() (and its caller) also move the unevictable page to unevictable list.
Therefore, in long term view, the page leak is not happend.

this explanation is enough?

thanks.