linux-kernel - Re: [PATCH v2] fix mlocked page counter mistmatch

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Date:	Thu, 5 Feb 2009 08:35:43 +0900
From:	MinChan Kim <minchan.kim@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux mm <linux-mm@...ck.org>,
	linux kernel <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH v2] fix mlocked page counter mistmatch

On Wed, Feb 04, 2009 at 07:28:19PM +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@...fujitsu.com>

Good. I will send adrew with your ACK agian.

> 
> 
> 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.

Thanks for clarification.
In long term view, you're right.

but My concern is that munlock[all] pathes always hold down of mmap_sem. 
After all, down_read_trylock always wil fail for such cases.

So, current task's mlocked pages only can be reclaimed 
by background or direct reclaim path if the task don't exit.

I think it can increase reclaim overhead unnecessary 
if there are lots of such tasks.

What's your opinion ?

> 
> this explanation is enough?
> 
> thanks.
> 

-- 
Kinds Regards
MinChan Kim

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