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

From: Dan Williams
Date: Fri Aug 16 2019 - 13:21:56 EST

On Fri, Aug 16, 2019 at 5:24 AM Jason Gunthorpe <jgg@xxxxxxxxxxxx> 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.