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

[David Rientjes]

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.  
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.

> 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);
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