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

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Date:	Thu, 5 Feb 2009 11:32:36 +0900
From:	MinChan Kim <barrioskmc@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	MinChan Kim <minchan.kim@...il.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

On Thu, Feb 5, 2009 at 11:17 AM, KOSAKI Motohiro
<kosaki.motohiro@...fujitsu.com> wrote:
>> > 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.

Sometime application of embedded can do it.
That's becuase they want deterministic page allocation in some situation.
However, It's not a matter in here.

>   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.

If we can drop mmap_sem in munlock path, I am happy, too.
Please, CCed me if you make a patch for it.

By that time, I will fold this issue. :)

>
> What do you think it?
>
>
>



-- 
Thanks,
MinChan Kim
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