[Linaro-mm-sig] how to avoid allocating or freeze MOVABLE memory in userspace

[Haojian Zhuang]

On Mon, Apr 16, 2012 at 9:55 PM, Christoph Lameter <cl at linux.com> wrote:
> On Sat, 14 Apr 2012, Haojian Zhuang wrote:
>
>> On Sat, Apr 14, 2012 at 2:27 AM, Christoph Lameter <cl at linux.com> wrote:
>> > On Fri, 13 Apr 2012, Haojian Zhuang wrote:
>> >
>> >> I have one question on memory migration. As we know, malloc() from
>> >> user app will allocate MIGRATE_MOVABLE pages. But if we want to use
>> >> this memory as DMA usage, we can't accept MIGRATE_MOVABLE type. Could
>> >> we change its behavior before DMA working?
>> >
>> > MIGRATE_MOVABLE works fine for DMA. If you keep a reference from a device
>> > driver to user pages then you will have to increase the page refcount
>> > which will in turn pin the page and make it non movable for as long as you
>> > keep the refcount.
>>
>> Hi Christoph,
>>
>> Thanks for your illustration. But it's a little abstract. Could you
>> give me a simple example
>> or show me the code?
>
> Run get_user_pages() on the memory you are interest in pinning. See how
> other drivers do that by looking up other use cases. F.e. ib_umem_get()
> does a similar thing.
>
>
Got it. And I think there's conflict in CMA.

For example, user process A malloc() memory, page->_count is 1. After
using get_user_pages()
in device driver for DMA usage, page->_count becomes 2.

If the page is in CMA region, it results migrate_pages() returns
-EAGAIN. But error handling in CMA is in below.

                ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA);
                if (ret == 0) {
                        bitmap_set(cma->bitmap, pageno, count);
                        break;
                } else if (ret != -EBUSY) {
                        goto error;
                }

Since EAGAIN doesn't equal to EBUSY, dma_alloc_from_contiguous()
aborts. Should dma_alloc_from_contiguous() handle EAGAIN?

Best Regards
Haojian