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

[Christoph Lameter <cl@linux.com>]

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.



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