linux-kernel - Re: [PATCH 1/6] sparc64: Add FORCE_MAX

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Date:   Mon, 24 Oct 2016 14:32:49 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     tushar.n.dave@...cle.com
Cc:     chris.hyser@...cle.com, sowmini.varadhan@...cle.com,
        vgupta@...opsys.com, robin.murphy@....com, jroedel@...e.de,
        egtvedt@...fundet.no, krzk@...nel.org, rkuo@...eaurora.org,
        sparclinux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] sparc64: Add FORCE_MAX_ZONEORDER and default to 13

From: tndave <tushar.n.dave@...cle.com>
Date: Mon, 24 Oct 2016 11:31:14 -0700

> 
> 
> On 10/24/2016 10:45 AM, David Miller wrote:
>> From: Tushar Dave <tushar.n.dave@...cle.com>
>> Date: Mon, 10 Oct 2016 11:12:02 -0700
>>
>>> From: Dave Kleikamp <dave.kleikamp@...cle.com>
>>>
>>> This change allows ATU (new IOMMU) in SPARC systems to request
>>> large (32M) contiguous memory during boot for creating IOTSB backing
>>> store.
>>>
>>> Signed-off-by: Dave Kleikamp <dave.kleikamp@...cle.com>
>>> Signed-off-by: Tushar Dave <tushar.n.dave@...cle.com>
>>
>> If you need 32MB allocations, wouldn't a max zone order of 12 be
>> sufficient?  (8K << 12 == 32MB)
> Yes, but config FORCE_MAX_ZONEORDER is actually maximum order plus
> one.
> For example, a value of 13 means that the largest free memory block is
> 2^12 pages. For 32MB we need 2^12 8K pages, hence FORCE_MAX_ZONEORDER
> must be MAX_ORDER + 1 i.e. 13.

Ok, thanks for explaining.