Nouveau - Aug 2019 - [PATCH 04/15] mm: remove the pgmap field from struct hmm_vma_walk

On Wed, Aug 14, 2019 at 6:28 AM Jason Gunthorpe <jgg at mellanox.com>
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.

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 at mellanox.com>
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).

Cheers,
Jérôme

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 at
mellanox.com> 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

On Thu, Aug 15, 2019 at 11:07 AM Jerome Glisse <jglisse at redhat.com>
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 at
mellanox.com> 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, as long as the driver remove is synchronized against HMM
operations via another mechanism then there is no need to take pagemap
references. Can you briefly describe what that other mechanism is?