Re: [PATCH v2] fix mlocked page counter mistmatch

[Lee Schermerhorn]

On Wed, 2009-02-04 at 19:28 +0900, KOSAKI Motohiro wrote:
> > 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@xxxxxxxxxxxxxx>
> 
> 
> 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.
> 

Also worth noting that down_read_trylock() does not "downgrade" the
semaphore.  It only tries to acquire it in read mode.  

As Kosaki-san says, try_to_unmap() doesn't normally hold the mmap_sem.
It needs to acquire it here to stabilize the vma [vm_flags] while
mlocking the pages.  This is the place where a page mapped in a
VM_LOCKED vma that vmscan found on the normal lru list--e.g., because we
couldn't isolate them in mlock_vma_page()--get marked mlocked, if not
already marked. mlock_vma_page() is a no-op if page is already mlocked.

If we successsfully acquire the mmap_sem and the vma is still VM_LOCKED,
we know that the page is mlocked and try_to_unmap() will return
SWAP_MLOCK.  This allows vmscan [shrink_page_list()] to move the page to
the unevictable list and not need to bother with it in subsequent scans
until it becomes munlocked.

Lee 



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