Re: [PATCH v6 11/26] mm: Allow compound zone device pages

Alistair Popple
2025-01-17T01:06:56Z

On Tue, Jan 14, 2025 at 03:59:31PM +0100, David Hildenbrand wrote:
> On 10.01.25 07:00, Alistair Popple wrote:
> > Zone device pages are used to represent various type of device memory
> > managed by device drivers. Currently compound zone device pages are
> > not supported. This is because MEMORY_DEVICE_FS_DAX pages are the only
> > user of higher order zone device pages and have their own page
> > reference counting.
> >
> > A future change will unify FS DAX reference counting with normal page
> > reference counting rules and remove the special FS DAX reference
> > counting. Supporting that requires compound zone device pages.
> >
> > Supporting compound zone device pages requires compound_head() to
> > distinguish between head and tail pages whilst still preserving the
> > special struct page fields that are specific to zone device pages.
> >
> > A tail page is distinguished by having bit zero being set in
> > page->compound_head, with the remaining bits pointing to the head
> > page. For zone device pages page->compound_head is shared with
> > page->pgmap.
> >
> > The page->pgmap field is common to all pages within a memory section.
> > Therefore pgmap is the same for both head and tail pages and can be
> > moved into the folio and we can use the standard scheme to find
> > compound_head from a tail page.
>
> The more relevant thing is that the pgmap field must be common to all pages
> in a folio, even if a folio exceeds memory sections (e.g., 128 MiB on x86_64
> where we have 1 GiB folios).

Thanks for pointing that out. I had assumed folios couldn't cross a memory
section. Obviously that is wrong so I've updated the commit message accordingly.

- Alistair

> > > Signed-off-by: Alistair Popple <apopple@nvidia.com>
> > Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> > Reviewed-by: Dan Williams <dan.j.williams@intel.com>
> >
> > ---
> >
> > Changes for v4:
> > - Fix build breakages reported by kernel test robot
> >
> > Changes since v2:
> >
> > - Indentation fix
> > - Rename page_dev_pagemap() to page_pgmap()
> > - Rename folio _unused field to _unused_pgmap_compound_head
> > - s/WARN_ON/VM_WARN_ON_ONCE_PAGE/
> >
> > Changes since v1:
> >
> > - Move pgmap to the folio as suggested by Matthew Wilcox
> > ---
>
> [...]
>
> > static inline bool folio_is_device_coherent(const struct folio *folio)
> > diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> > index 29919fa..61899ec 100644
> > --- a/include/linux/migrate.h
> > +++ b/include/linux/migrate.h
> > @@ -205,8 +205,8 @@ struct migrate_vma {
> > unsigned long end;
> > /*
> > - * Set to the owner value also stored in page->pgmap->owner for
> > - * migrating out of device private memory. The flags also need to
> > + * Set to the owner value also stored in page_pgmap(page)->owner
> > + * for migrating out of device private memory. The flags also need to
> > * be set to MIGRATE_VMA_SELECT_DEVICE_PRIVATE.
> > * The caller should always set this field when using mmu notifier
> > * callbacks to avoid device MMU invalidations for device private
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index df8f515..54b59b8 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -129,8 +129,11 @@ struct page {
> > unsigned long compound_head; /* Bit zero is set */
> > };
> > struct { /* ZONE_DEVICE pages */
> > - /** @pgmap: Points to the hosting device page map. */
> > - struct dev_pagemap *pgmap;
> > + /*
> > + * The first word is used for compound_head or folio
> > + * pgmap
> > + */
> > + void *_unused_pgmap_compound_head;
> > void *zone_device_data;
> > /*
> > * ZONE_DEVICE private pages are counted as being
> > @@ -299,6 +302,7 @@ typedef struct {
> > * @_refcount: Do not access this member directly. Use folio_ref_count()
> > * to find how many references there are to this folio.
> > * @memcg_data: Memory Control Group data.
> > + * @pgmap: Metadata for ZONE_DEVICE mappings
> > * @virtual: Virtual address in the kernel direct map.
> > * @_last_cpupid: IDs of last CPU and last process that accessed the folio.
> > * @_entire_mapcount: Do not use directly, call folio_entire_mapcount().
> > @@ -337,6 +341,7 @@ struct folio {
> > /* private: */
> > };
> > /* public: */
> > + struct dev_pagemap *pgmap;
>
> Agreed, that should work.
>
> Acked-by: David Hildenbrand <david@redhat.com>
>
> --
> Cheers,
>
> David / dhildenb
>