mm/memremap: add ZONE_DEVICE support for compound pages

Add a new @vmemmap_shift property for struct dev_pagemap which specifies
that a devmap is composed of a set of compound pages of order
@vmemmap_shift, instead of base pages.  When a compound page devmap is
requested, all but the first page are initialised as tail pages instead
of order-0 pages.

For certain ZONE_DEVICE users like device-dax which have a fixed page
size, this creates an opportunity to optimize GUP and GUP-fast walkers,
treating it the same way as THP or hugetlb pages.

Additionally, commit 7118fc2906e2 ("hugetlb: address ref count racing in
prep_compound_gigantic_page") removed set_page_count() because the
setting of page ref count to zero was redundant.  devmap pages don't
come from page allocator though and only head page refcount is used for
compound pages, hence initialize tail page count to zero.

Link: https://lkml.kernel.org/r/20211202204422.26777-5-joao.m.martins@oracle.com
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Jane Chu <jane.chu@oracle.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index c0e9d35889e8..61a6a0e27359 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -99,6 +99,11 @@ struct dev_pagemap_ops {
  * @done: completion for @internal_ref
  * @type: memory type: see MEMORY_* in memory_hotplug.h
  * @flags: PGMAP_* flags to specify defailed behavior
+ * @vmemmap_shift: structural definition of how the vmemmap page metadata
+ *      is populated, specifically the metadata page order.
+ *	A zero value (default) uses base pages as the vmemmap metadata
+ *	representation. A bigger value will set up compound struct pages
+ *	of the requested order value.
  * @ops: method table
  * @owner: an opaque pointer identifying the entity that manages this
  *	instance.  Used by various helpers to make sure that no
@@ -114,6 +119,7 @@ struct dev_pagemap {
 	struct completion done;
 	enum memory_type type;
 	unsigned int flags;
+	unsigned long vmemmap_shift;
 	const struct dev_pagemap_ops *ops;
 	void *owner;
 	int nr_range;
@@ -130,6 +136,11 @@ static inline struct vmem_altmap *pgmap_altmap(struct dev_pagemap *pgmap)
 	return NULL;
 }
 
+static inline unsigned long pgmap_vmemmap_nr(struct dev_pagemap *pgmap)
+{
+	return 1 << pgmap->vmemmap_shift;
+}
+
 #ifdef CONFIG_ZONE_DEVICE
 void *memremap_pages(struct dev_pagemap *pgmap, int nid);
 void memunmap_pages(struct dev_pagemap *pgmap);