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

[Will Deacon]

On Fri, Apr 08, 2016 at 09:50:43AM -0700, Doug Anderson wrote:
> On Fri, Apr 8, 2016 at 6:07 AM, Will Deacon <will.deacon@xxxxxxx> wrote:
> > On Tue, Apr 05, 2016 at 10:03:32AM -0700, Doug Anderson wrote:
> >> On Tue, Mar 29, 2016 at 10:02 AM, Will Deacon <will.deacon@xxxxxxx> wrote:
> >> > On Mon, Mar 28, 2016 at 02:32:11PM +0800, Yong Wu wrote:
> >> >> @@ -213,13 +215,16 @@ static struct page **__iommu_dma_alloc_pages(unsigned int count, gfp_t gfp)
> >> >>               /*
> >> >>                * Higher-order allocations are a convenience rather
> >> >>                * than a necessity, hence using __GFP_NORETRY until
> >> >> -              * falling back to single-page allocations.
> >> >> +              * falling back to min size allocations.
> >> >>                */
> >> >> -             for (order = min_t(unsigned int, order, __fls(count));
> >> >> -                  order > 0; order--) {
> >> >> -                     page = alloc_pages(gfp | __GFP_NORETRY, order);
> >> >> +             for (order = min_t(int, order, __fls(count));
> >> >> +                  order >= min_order; order--) {
> >> >> +                     page = alloc_pages((order == min_order) ? gfp :
> >> >> +                                        gfp | __GFP_NORETRY, order);
> >> >>                       if (!page)
> >> >>                               continue;
> >> >> +                     if (!order)
> >> >> +                             break;
> >> >
> >> > Isn't this handled by the loop condition?
> >>
> >> He changed the loop condition to be ">= min_order" instead of "> 0",
> >> so now we can get here with an order == 0.  This makes sense because
> >> when min_order is not 0 you still want to run the code to split the
> >> pages and it is sane not to duplicate that below.
> >>
> >> Maybe I'm misunderstanding, though.  Perhaps you can explain how you
> >> think this code should look?
> >
> > My reading of the code was that we require order >= min_order to enter
> > the loop. Given that order doesn't change between the loop header and the
> > if (!order) check, then that must mean we can enter the loop body with
> > order == 0 and order >= min_order, which means that min_order is allowed
> > to be negative. That feels weird.
> >
> > 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.

Will