linux-kernel - Re: [PATCH] arm64: fix MAX

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Message-ID: <xa1tfvj01z57.fsf@mina86.com>
Date:	Thu, 19 Jun 2014 21:24:04 +0200
From:	Michal Nazarewicz <mina86@...a86.com>
To:	Mark Salter <msalter@...hat.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: fix MAX_ORDER for 64K pagesize

On Thu, Jun 19 2014, Mark Salter <msalter@...hat.com> wrote:
> On Tue, 2014-06-17 at 20:32 +0200, Michal Nazarewicz wrote:
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 5dba293..6e657ce 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -801,7 +801,15 @@ void __init init_cma_reserved_pageblock(struct page *page)
>>  
>>  	set_page_refcounted(page);
>>  	set_pageblock_migratetype(page, MIGRATE_CMA);
>> -	__free_pages(page, pageblock_order);
>> +	if (pageblock_order > MAX_ORDER) {
>> +		struct page *subpage = p;
>> +		unsigned count = 1 << (pageblock_order - MAX_ORDER);
>> +		do {
>> +			__free_pages(subpage, pageblock_order);
>                                                ^^^^^^^
>                                                MAX_ORDER

D'oh!  I'll send a revised patch.

>> +		} while (subpage += MAX_ORDER_NR_PAGES, --count);
>> +	} else {
>> +		__free_pages(page, pageblock_order);
>> +	}
>>  	adjust_managed_page_count(page, pageblock_nr_pages);
>>  }
>>  #endif
>> --------- >8 ---------------------------------------------------------
>> 
>> Thoughts?  This has not been tested and I think it may cause performance
>> degradation in some cases since pageblock_order is not always
>> a constant, so the comparison may end up not being stripped away even on
>> systems where it's always false.

> This works with the above tweak. So it fixes the problm here, but I was
> not sure if we'd get bitten elsewhere by pageblock_order > MAX_ORDER.

This is always a possibility, but in such cases, it's a bug in CMA.
I've tried to keep in mind that pageblock_order may be greater than
MAX_ORDER when writing CMA, but I've never tested on such a system.

> It will be slower, but does it only gets called a few time at most at
> boot time, right?

Yes.  The performance degradation should be negligible since
init_cma_reserved is hardly a critical path and is called at most
MAX_CMA_AREAS times which by default is 8.  And I mean it will be slower
because it will have to perform a branch.


-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
ooo +--<mpn@...gle.com>--<xmpp:mina86@...ber.org>--ooO--(_)--Ooo--
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