Re: [PATCH v2 1/2] dma/iommu: Add pgsize_bitmap confirmation in __iommu_dma_alloc_pages

Doug Anderson
2016-04-08T17:34:14Z

Hi,

On Fri, Apr 8, 2016 at 10:30 AM, Will Deacon <will.deacon@arm.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