linux-kernel - Re: [PATCH v2 1/2] dma/iommu: Add pgsize_bitmap confirmation in __iommu_dma_alloc

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Message-ID: <CAD=FV=XbL6gbdmX62Q3QJQ6R-rC7cCd44RLPgK3kvhAC49BJjA@mail.gmail.com>
Date:	Fri, 8 Apr 2016 10:34:10 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Will Deacon <will.deacon@....com>
Cc:	Yong Wu <yong.wu@...iatek.com>, Joerg Roedel <joro@...tes.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Robin Murphy <robin.murphy@....com>,
	Daniel Kurtz <djkurtz@...gle.com>,
	Tomasz Figa <tfiga@...gle.com>, Arnd Bergmann <arnd@...db.de>,
	Lucas Stach <l.stach@...gutronix.de>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	"moderated list:ARM/Mediatek SoC support" 
	<linux-mediatek@...ts.infradead.org>, srv_heupstream@...iatek.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	iommu@...ts.linux-foundation.org
Subject: Re: [PATCH v2 1/2] dma/iommu: Add pgsize_bitmap confirmation in __iommu_dma_alloc_pages

Hi,

On Fri, Apr 8, 2016 at 10:30 AM, Will Deacon <will.deacon@....com> wrote:
>> > Am I barking up the wrong tree?
>>
>> I don't think min_order can be negative.  Certainly we could enter the
>> loop with order == 0 and min_order == 0, though.
>
> ... and in that case, PageCompound will be false, and we'll call split_page
> which won't do anything, so we break out.
>
>>
>> Some examples:
>>
>> order = 0, min_order = 0
>> -> Want alloc_pages _without_ __GFP_NORETRY.  OK
>> -> If alloc_pages fails, return NULL.  OK
>> -> If alloc pages succeeds, don't need splitting since single page.  OK
>
> [...]
>
>> I think those are all right.  Did I mess up?  You could certainly
>> structure the loop in a different way but you need to make sure you
>> handle all of those cases.  If you have an alternate structure that
>> handles all those, let's consider it.
>
> Right, I don't think the code is broken, I just think the !order check is
> confusing and not needed.

Ah ha!  Got it.  I didn't dig into split_page() to see that it was a
no-op when "order == 0".  I just know that the old code didn't call
split_page() with order == 0 so I assumed that was wise to keep.  If
we don't need to keep that then agreed that the "if" test can simply
be removed.  :)

-Doug