Re: [PATCH 08/12] gup: Don't allow FOLL_LONGTERM pinning of FS DAX pages

[Dan Williams <dan.j.williams@intel.com>]

Dan Williams wrote:
> Alistair Popple wrote:
> > Longterm pinning of FS DAX pages should already be disallowed by
> > various pXX_devmap checks. However a future change will cause these
> > checks to be invalid for FS DAX pages so make
> > folio_is_longterm_pinnable() return false for FS DAX pages.
> > 
> > Signed-off-by: Alistair Popple <apopple@nvidia.com>
> > ---
> >  include/linux/memremap.h | 11 +++++++++++
> >  include/linux/mm.h       |  4 ++++
> >  2 files changed, 15 insertions(+)
> > 
> > diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> > index 14273e6..6a1406a 100644
> > --- a/include/linux/memremap.h
> > +++ b/include/linux/memremap.h
> > @@ -187,6 +187,17 @@ static inline bool folio_is_device_coherent(const struct folio *folio)
> >  	return is_device_coherent_page(&folio->page);
> >  }
> >  
> > +static inline bool is_device_dax_page(const struct page *page)
> > +{
> > +	return is_zone_device_page(page) &&
> > +		page_dev_pagemap(page)->type == MEMORY_DEVICE_FS_DAX;
> > +}
> > +
> > +static inline bool folio_is_device_dax(const struct folio *folio)
> > +{
> > +	return is_device_dax_page(&folio->page);
> > +}
> > +
> >  #ifdef CONFIG_ZONE_DEVICE
> >  void zone_device_page_init(struct page *page);
> >  void *memremap_pages(struct dev_pagemap *pgmap, int nid);
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index ae6d713..935e493 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1989,6 +1989,10 @@ static inline bool folio_is_longterm_pinnable(struct folio *folio)
> >  	if (folio_is_device_coherent(folio))
> >  		return false;
> >  
> > +	/* DAX must also always allow eviction. */
> > +	if (folio_is_device_dax(folio))
> 
> Why is this called "folio_is_device_dax()" when the check is for fsdax?
> 
> I would expect:
> 
> if (folio_is_fsdax(folio))
> 	return false;
> 
> ...and s/device_dax/fsdax/ for the rest of the helpers.

Specifically devdax is ok to allow longterm pinning since it is
statically allocated. fsdax is the only ZONE_DEVICE mode where there is
a higher-level allocator that does not support a 3rd party the block its
operations indefinitely with a pin. So this needs to be explicit for
that case.