[SRU][P][PATCH 1/6] mm: Provide address mask in struct follow_pfnmap_args

[Mitchell Augustin]

From: Alex Williamson <alex.williamson at redhat.com>

BugLink: https://bugs.launchpad.net/bugs/2111861

follow_pfnmap_start() walks the page table for a given address and
fills out the struct follow_pfnmap_args in pfnmap_args_setup().
The address mask of the page table level is already provided to this
latter function for calculating the pfn.  This address mask can also
be useful for the caller to determine the extent of the contiguous
mapping.

For example, vfio-pci now supports huge_fault for pfnmaps and is able
to insert pud and pmd mappings.  When we DMA map these pfnmaps, ex.
PCI MMIO BARs, we iterate follow_pfnmap_start() to get each pfn to test
for a contiguous pfn range.  Providing the mapping address mask allows
us to skip the extent of the mapping level.  Assuming a 1GB pud level
and 4KB page size, iterations are reduced by a factor of 256K.  In wall
clock time, mapping a 32GB PCI BAR is reduced from ~1s to <1ms.

Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: David Hildenbrand <david at redhat.com>
Cc: linux-mm at kvack.org
Reviewed-by: Peter Xu <peterx at redhat.com>
Reviewed-by: "Mitchell Augustin" <mitchell.augustin at canonical.com>
Tested-by: "Mitchell Augustin" <mitchell.augustin at canonical.com>
Reviewed-by: Jason Gunthorpe <jgg at nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson at redhat.com>
(cherry picked from commit 62fb8adc43afad5fa1c9cadc6f3a8e9fb72af194)
Signed-off-by: Mitchell Augustin <mitchell.augustin at canonical.com>
---
 include/linux/mm.h | 2 ++
 mm/memory.c        | 1 +
 2 files changed, 3 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1f80baddacc5..72dbec7ed901 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2423,11 +2423,13 @@ struct follow_pfnmap_args {
 	 * Outputs:
 	 *
 	 * @pfn: the PFN of the address
+	 * @addr_mask: address mask covering pfn
 	 * @pgprot: the pgprot_t of the mapping
 	 * @writable: whether the mapping is writable
 	 * @special: whether the mapping is a special mapping (real PFN maps)
 	 */
 	unsigned long pfn;
+	unsigned long addr_mask;
 	pgprot_t pgprot;
 	bool writable;
 	bool special;
diff --git a/mm/memory.c b/mm/memory.c
index 421d76ca3831..cab50d7c317f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6475,6 +6475,7 @@ static inline void pfnmap_args_setup(struct follow_pfnmap_args *args,
 	args->lock = lock;
 	args->ptep = ptep;
 	args->pfn = pfn_base + ((args->address & ~addr_mask) >> PAGE_SHIFT);
+	args->addr_mask = addr_mask;
 	args->pgprot = pgprot;
 	args->writable = writable;
 	args->special = special;
-- 
2.43.0