[PATCH 04/15] mm: remove the pgmap field from struct hmm_vma_walk

Dan Williams dan.j.williams at intel.com
Fri Aug 16 17:21:41 UTC 2019

On Fri, Aug 16, 2019 at 5:24 AM Jason Gunthorpe <jgg at mellanox.com> wrote:
> On Thu, Aug 15, 2019 at 08:54:46PM -0700, Dan Williams wrote:
> > > However, this means we cannot do any processing of ZONE_DEVICE pages
> > > outside the driver lock, so eg, doing any DMA map that might rely on
> > > MEMORY_DEVICE_PCI_P2PDMA has to be done in the driver lock, which is
> > > a bit unfortunate.
> >
> > Wouldn't P2PDMA use page pins? Not needing to hold a lock over
> > ZONE_DEVICE page operations was one of the motivations for plumbing
> > get_dev_pagemap() with a percpu-ref.
> hmm_range_fault() doesn't use page pins at all, so if a ZONE_DEVICE
> page comes out of it then it needs to use another locking pattern.
> If I follow it all right:
> We can do a get_dev_pagemap inside the page_walk and touch the pgmap,
> or we can do the 'device mutex && retry' pattern and touch the pgmap
> in the driver, under that lock.
> However in all cases the current get_dev_pagemap()'s in the page walk
> are not necessary, and we can delete them.

Yes, as long as 'struct page' instances resulting from that lookup are
not passed outside of that lock.

More information about the amd-gfx mailing list