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

[Jason Gunthorpe]

On Thu, Aug 15, 2019 at 02:03:25PM -0400, Jerome Glisse wrote:
> On Wed, Aug 14, 2019 at 07:48:28AM -0700, Dan Williams wrote:
> > On Wed, Aug 14, 2019 at 6:28 AM Jason Gunthorpe <jgg@xxxxxxxxxxxx> wrote:
> > >
> > > On Wed, Aug 14, 2019 at 09:38:54AM +0200, Christoph Hellwig wrote:
> > > > On Tue, Aug 13, 2019 at 06:36:33PM -0700, Dan Williams wrote:
> > > > > Section alignment constraints somewhat save us here. The only example
> > > > > I can think of a PMD not containing a uniform pgmap association for
> > > > > each pte is the case when the pgmap overlaps normal dram, i.e. shares
> > > > > the same 'struct memory_section' for a given span. Otherwise, distinct
> > > > > pgmaps arrange to manage their own exclusive sections (and now
> > > > > subsections as of v5.3). Otherwise the implementation could not
> > > > > guarantee different mapping lifetimes.
> > > > >
> > > > > That said, this seems to want a better mechanism to determine "pfn is
> > > > > ZONE_DEVICE".
> > > >
> > > > So I guess this patch is fine for now, and once you provide a better
> > > > mechanism we can switch over to it?
> > >
> > > What about the version I sent to just get rid of all the strange
> > > put_dev_pagemaps while scanning? Odds are good we will work with only
> > > a single pagemap, so it makes some sense to cache it once we find it?
> > 
> > Yes, if the scan is over a single pmd then caching it makes sense.
> 
> Quite frankly an easier an better solution is to remove the pagemap
> lookup as HMM user abide by mmu notifier it means we will not make
> use or dereference the struct page so that we are safe from any
> racing hotunplug of dax memory (as long as device driver using hmm
> do not have a bug).

Yes, I also would prefer to drop the confusing checks entirely -
Christoph can you resend this patch?

Thanks,
Jason