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>

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7b1068ddcbb70b732a4f2843e6b55fd73cc3e320..92b30dba7e38ea2b3103fc889116d68115454ab4 100644 (file)
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2417,11 +2417,13 @@ 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 b4d3d4893267c8c9007a2b6e5419136ece85b59a..68aa0f11633eb29adbfcb2e2cd2ed2e0d87ce94f 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -6488,6 +6488,7 @@ 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;