linux-kernel - Re: [PATCH] take pageout refcount into account for remove_exclusive_swap

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Message-Id: <482B9CCA.9060203@mxp.nes.nec.co.jp>
Date:	Thu, 15 May 2008 11:15:38 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	Lee Schermerhorn <Lee.Schermerhorn@...com>
CC:	Rik van Riel <riel@...hat.com>, akpm@...ux-foundation.org,
	kosaki.motohiro@...fujitsu.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] take pageout refcount into account for remove_exclusive_swap_page

On 2008/05/14 4:02 +0900, Lee Schermerhorn wrote:
> On Tue, 2008-05-13 at 14:09 -0400, Rik van Riel wrote:
>> On Tue, 13 May 2008 13:43:55 -0400
>> Lee Schermerhorn <Lee.Schermerhorn@...com> wrote:
>>
>>> Or, more generally, 2 + N, for an anon page that is mapped [must be
>>> read-only, right?] by N processes.  This can happen after, e.g., fork().
>>> Looks like this patch handles the condition just fine, but you might
>>> want to reflect this in the comment.
>> No, this patch only removes a page from the swap cache that is mapped
>> by one process.  The function page_mapped() returns either 1 or 0, not
>> the same as page_mapcount().
> 
> Duh!  I was reading "page_mapcount()", 'cause that's what I've been
> considering using for this purpose.
> 
>>  
>> I am not sure if trying to handle swap cache pages that are mapped by
>> multiple processes could get us into other corner cases and think that
>> we should probably try to stick to the safe thing for now.

I think it would be better to add a comment of
remove_exclusive_swap_page_ref() that it doesn't handle
swap caches that are mapped by multiple processes for safty.

> 
> OK.  I can test the more general case down the road.
> 
>> Besides, shouldn't anonymous shared pages be COW and relatively rare?
> 
> Well, all anon pages are shared right after fork(), right?  They only
> become private once they've been written to.  I don't have a feel for
> the relative numbers of shared anon vs COWed anon--either in general or
> in the swap cache.
> 
>>> Now, I think I can use this to try to remove anon pages from the swap
>>> cache when they're mlocked.
> 
> I suppose I can just go ahead and use this version with my stress load
> and count the times when I could have freed the swap cache entry, but
> didn't because it's mapped in multiple address spaces.  Later ... :)
> 

Lee, Rik's version looks good to me except mlocked case, but
do you have any plan to handle these cases?


>>>> + */
>>>> +int remove_exclusive_swap_page_ref(struct page *page)
>>>> +{
>>>> +	return remove_exclusive_swap_page_count(page, 2 + page_mapped(page));
>>>> +}
>>>> +
>>>> +/*
>>>>   * Free the swap entry like above, but also try to
>>>>   * free the page cache entry if it is the last user.
>>>>   */
>>>>
>>>> All Rights Reversed
>>
> 
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