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

[Tomasz Figa]

Hi David,

On Tue, Mar 17, 2015 at 8:32 AM, David Rientjes <rientjes at google.com> wrote:
> On Mon, 16 Mar 2015, Tomasz Figa wrote:
>
>> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
>> index 83cd5ac..f081e9e 100644
>> --- a/arch/arm/mm/dma-mapping.c
>> +++ b/arch/arm/mm/dma-mapping.c
>> @@ -1145,18 +1145,31 @@ static struct page **__iommu_alloc_buffer(struct device *dev, size_t size,
>>       }
>>
>>       /*
>> -      * IOMMU can map any pages, so himem can also be used here
>> +      * IOMMU can map any pages, so himem can also be used here.
>> +      * We do not want OOM killer to be invoked as long as we can fall back
>> +      * to single pages, so we use __GFP_NORETRY for positive orders.
>>        */
>> -     gfp |= __GFP_NOWARN | __GFP_HIGHMEM;
>> +     gfp |= __GFP_NOWARN | __GFP_HIGHMEM | __GFP_NORETRY;
>>
>>       while (count) {
>> -             int j, order = __fls(count);
>> +             int j, order;
>>
>> -             pages[i] = alloc_pages(gfp, order);
>> -             while (!pages[i] && order)
>> -                     pages[i] = alloc_pages(gfp, --order);
>> -             if (!pages[i])
>> -                     goto error;
>> +             for (order = __fls(count); order; --order) {
>> +                     /* Will not trigger OOM. */
>> +                     pages[i] = alloc_pages(gfp, order);
>> +                     if (pages[i])
>> +                             break;
>> +             }
>> +
>> +             if (!pages[i]) {
>> +                     /*
>> +                      * Fall back to single page allocation.
>> +                      * Might invoke OOM killer as last resort.
>> +                      */
>> +                     pages[i] = alloc_pages(gfp & ~__GFP_NORETRY, 0);
>> +                     if (!pages[i])
>> +                             goto error;
>> +             }
>>
>>               if (order) {
>>                       split_page(pages[i], order);
>
> I think this makes sense, but the problem is the unconditional setting and
> clearing of __GFP_NORETRY.  Strictly speaking, gfp may already have
> __GFP_NORETRY set when calling this function so it would be better to do
> the loop with alloc_pages(gfp | __GFP_NORETRY, order) and then the
> fallback as alloc_page(gfp).

Good point. I'll change it to that in next version.

Best regards,
Tomasz