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

Dan Williams
2024-09-27T02:54:31Z

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.