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

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Date:	Thu,  5 Feb 2009 11:17:46 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	MinChan Kim <minchan.kim@...il.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	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>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>
Subject: Re: [PATCH v2] fix mlocked page counter mistmatch

> > 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 ?

I have 2 comment.

1. typical application never munlock()ed at all.
   and exit() path is already efficient.
   then, I don't like hacky apploach.
2. I think we should drop mmap_sem holding in munlock path in the future.
   at that time, this issue disappear automatically.
   it's clean way more.

What do you think it?


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