linux-kernel - Re: [RFC][PATCH 3/3] alloc contig pages with migration.

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Message-ID: <AANLkTimpVWwH=znGkG8zEPBcYxq-+UR+7mN29f-RK7d=@mail.gmail.com>
Date:	Mon, 18 Oct 2010 14:52:19 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 3/3] alloc contig pages with migration.

On Mon, Oct 18, 2010 at 2:31 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> On Mon, 18 Oct 2010 14:18:52 +0900
> Minchan Kim <minchan.kim@...il.com> wrote:
>> >
>> >> > + *
>> >> > + * Search an area of @size in the physical memory map and checks wheter
>> >>
>> >> Typo
>> >> whether
>> >>
>> >> > + * we can create a contigous free space. If it seems possible, try to
>> >> > + * create contigous space with page migration. If no_search==true, we just try
>> >> > + * to allocate [hint, hint+size) range of pages as contigous block.
>> >> > + *
>> >> > + * Returns a page of the beginning of contiguous block. At failure, NULL
>> >> > + * is returned. Each page in the area is set to page_count() = 1. Because
>> >>
>> >> Why do you mention page_count() = 1?
>> >> Do users of this function have to know it?
>> >
>> > A user can free any page within the range for his purpose.
>>
>> I think it's not a good idea if we allow handling of page by page, not
>> for page-chunk requested by user.
>> By mistake, free_contig_pages could have a trouble to free pages.
>> Why do you support the feature?  Do you have any motivation?
>>
> No big motivation.
>
> Usual pages are set up by prep_compund_page(page, order), but it is pages smaller
> than MAX_ORDER.  Then, I called prep_new_page() one by one.
> And I don't think some new prep_xxxx_page() is required.
>
> If you requests, ok, I'll add one.

Maybe we are talking another thing.

My question is why you noticed "page_count() == 1" in function description.
So your answer was for user to free some pages within big contiguous page.
Then, my concern is that if you didn't mentioned page_count() == 1 in
description, anonymous user will use just alloc_contig_pages and
free_contig_pages. That's enough for current our requirement. But
since you mentioned page_count() == 1 and you want for user to free
some pages within big contiguous page, anonymous user who isn't expert
in mm or careless people can free pages _freely_. It could make BUG
easily(free_contig_pages can free the page which is freed by user's
put_page).

So if there isn't strong cause, I hope removing the mention for
preventing careless API usage.

>
> Thanks,
> -Kame
>
>



-- 
Kind regards,
Minchan Kim
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