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

From: John Hubbard
Date: Wed May 01 2024 - 13:33:01 EST


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