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

Yong Wu
2016-04-11T07:40:20Z

On Fri, 2016-04-08 at 10:34 -0700, Doug Anderson wrote:
> 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

Hi Will, Doug,

Thanks very much for review this patch, and Thanks Robin's work,
Currently this one is obsoleted since it's included in [1].

[1]:http://lists.linuxfoundation.org/pipermail/iommu/2016-April/016402.html