Re: [PATCH v2] ARM: mm: Do not invoke OOM for higher order IOMMU DMA allocations

[Ritesh Harjani]

On Thu, Mar 26, 2015 at 6:04 AM, David Rientjes <rientjes@xxxxxxxxxx> wrote:
> On Thu, 26 Mar 2015, Ritesh Harjani wrote:
>
>> > diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
>> > index 83cd5ac..3f1ac51 100644
>> > --- a/arch/arm/mm/dma-mapping.c
>> > +++ b/arch/arm/mm/dma-mapping.c
>> > @@ -1150,13 +1150,28 @@ static struct page **__iommu_alloc_buffer(struct device *dev, size_t size,
>> >         gfp |= __GFP_NOWARN | __GFP_HIGHMEM;
>> >
>> >         while (count) {
>> > -               int j, order = __fls(count);
>> > +               int j, order;
>> > +
>> > +               for (order = __fls(count); order > 0; --order) {
>> > +                       /*
>> > +                        * We do not want OOM killer to be invoked as long
>> > +                        * as we can fall back to single pages, so we force
>> > +                        * __GFP_NORETRY for orders higher than zero.
>> > +                        */
>> > +                       pages[i] = alloc_pages(gfp | __GFP_NORETRY, order);
>> > +                       if (pages[i])
>> > +                               break;
>> > +               }
>> >
>> > -               pages[i] = alloc_pages(gfp, order);
>> > -               while (!pages[i] && order)
>> > -                       pages[i] = alloc_pages(gfp, --order);
>> > -               if (!pages[i])
>> > -                       goto error;
>> > +               if (!pages[i]) {
>> > +                       /*
>> > +                        * Fall back to single page allocation.
>> > +                        * Might invoke OOM killer as last resort.
>> > +                        */
>> > +                       pages[i] = alloc_pages(gfp, 0);
>> I think down the code in this while loop, i & count is being
>> calculated based on the "order" of allocation in the current
>> iteration.
>> Since value of order will be automatically 0 here if (!pages[i]) is
>> true then, why hard code order to value of 0 here.
>> Comment clearly says what this code is doing right?
>>
>
> Gcc is smart enough to know that order == 0 here, the code generation on
> arm will be the same, so this is only a matter of how the source looks.

Agreed.
> To me, it doesn't make a lot of sense to write it as alloc_pages(gfp,
> order) when order is always equal to 0.  I think it's clearer the way that
> Tomasz wrote it.

Ok
>
>> I know it is just a minor thing. Don't know if it is relevant.
>>
>> > +                       if (!pages[i])
>> > +                               goto error;
>> > +               }
>> >
>> >                 if (order) {
>> >                         split_page(pages[i], order);

Thanks
Ritesh
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