Re: [PATCH 05/12] mm: alloc_contig_range() added

["Michal Nazarewicz" <mina86@mina86.com>]

On Thu, 31 Mar 2011 22:28:42 +0200, Dave Hansen <dave@linux.vnet.ibm.com>  
wrote:

> On Thu, 2011-03-31 at 18:26 +0200, Michal Nazarewicz wrote:
>> > On Thu, 2011-03-31 at 15:16 +0200, Marek Szyprowski wrote:
>> >> +       ret = 0;
>> >> +       while (!PageBuddy(pfn_to_page(start & (~0UL << ret))))
>> >> +               if (WARN_ON(++ret >= MAX_ORDER))
>> >> +                       return -EINVAL;
>>
>> On Thu, 31 Mar 2011 18:02:41 +0200, Dave Hansen wrote:
>> > Holy cow, that's dense.  Is there really no more straightforward way  
>> to
>> > do that?
>>
>> Which part exactly is dense?  What would be qualify as a more
>> straightforward way?
>
> I'm still not 100% sure what it's trying to do.  It looks like it
> attempts to check all of "start"'s buddy pages.

No.  I'm going up through parents.  This is because even though start
falls in a free block (ie. one that page allocator tracks), the actual
page that is in buddy system is larger then start and this loop looks
for beginning of that page.

> int order;
> for (order = 0; order <= MAX_ORDER; order++) {
> 	unsigned long buddy_pfn = find_buddy(start, order);
> 	struct page *buddy = pfn_to_page(buddy_pfn);
> 	if (PageBuddy(buddy)
> 		break;
> 	WARN();
> 	return -EINVAL;
> }

The WARN() and return would have to be outside of the loop and, as I
described, instead of find_buddy() something like find_parent() would
have to be used.

> I'm wondering also if you can share some code with __rmqueue().

Doubtful since start does not (have to) point to a page that is tracked
by page allocator but a page inside such a page.

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michal "mina86" Nazarewicz    (o o)
ooo +-----<email/xmpp: mnazarewicz@google.com>-----ooO--(_)--Ooo--

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