RE: how to avoid allocating or freeze MOVABLE memory in userspace

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hello,

On Wednesday, April 18, 2012 9:37 AM Haojian Zhuang wrote:

> On Mon, Apr 16, 2012 at 9:55 PM, Christoph Lameter <cl@linux.com> wrote:
> > On Sat, 14 Apr 2012, Haojian Zhuang wrote:
> >
> >> On Sat, Apr 14, 2012 at 2:27 AM, Christoph Lameter <cl@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?
> 

I've checked again and it is really not possible for migrate_pages() to return -EAGAIN, 
so the CMA code is correct. The only special case which needs retry is -EBUSY.

Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center


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