Re: [RFC] RDMA/umem: pin_user_pages*() can temporarily fail due to migration glitches

John Hubbard
2024-05-01T17:32:51Z

On 5/1/24 5:10 AM, Jason Gunthorpe wrote:
> On Tue, Apr 30, 2024 at 10:10:43PM -0700, Christoph Hellwig wrote:
...
>> This doesn't make sense. IFF a blind retry is all that is needed it
>> should be done in the core functionality. I fear it's not that easy,
>> though.

So do I. :)

>
> +1
>
> This migration retry weirdness is a GUP issue, it needs to be solved
> in the mm not exposed to every pin_user_pages caller.
>
> If it turns out ZONE_MOVEABLE pages can't actually be reliably moved
> then it is pretty broken..
>

OK, I'll work on finding out what is temporarily elevating the refcount
and preventing the migration. And see where that leads.

thanks,
--
John Hubbard
NVIDIA