mm: Provide address mask in struct follow_pfnmap_args

JIRA: https://issues.redhat.com/browse/RHEL-85587

commit 62fb8ad
Author: Alex Williamson <alex.williamson@redhat.com>
Date:   Tue Feb 18 15:22:05 2025 -0700

    mm: Provide address mask in struct follow_pfnmap_args

    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@linux-foundation.org>
    Cc: David Hildenbrand <david@redhat.com>
    Cc: linux-mm@kvack.org
    Reviewed-by: Peter Xu <peterx@redhat.com>
    Reviewed-by: Mitchell Augustin <mitchell.augustin@canonical.com>
    Tested-by: Mitchell Augustin <mitchell.augustin@canonical.com>
    Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
    Acked-by: David Hildenbrand <david@redhat.com>
    Link: https://lore.kernel.org/r/20250218222209.1382449-6-alex.williamson@redhat.com
    Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Author: Alex Williamson

include/linux/mm.h

@@ -2405,11 +2405,13 @@ 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;

mm/memory.c

@@ -6353,6 +6353,7 @@ 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;