mm: page_alloc: skip over regions of invalid pfns where possible

When using a sparse memory model memmap_init_zone() when invoked with
the MEMMAP_EARLY context will skip over pages which aren't valid - ie.
which aren't in a populated region of the sparse memory map.  However if
the memory map is extremely sparse then it can spend a long time
linearly checking each PFN in a large non-populated region of the memory
map & skipping it in turn.

When CONFIG_HAVE_MEMBLOCK_NODE_MAP is enabled, we have sufficient
information to quickly discover the next valid PFN given an invalid one
by searching through the list of memory regions & skipping forwards to
the first PFN covered by the memory region to the right of the
non-populated region.  Implement this in order to speed up
memmap_init_zone() for systems with extremely sparse memory maps.

James said "I have tested this patch on a virtual model of a Samurai CPU
with a sparse memory map.  The kernel boot time drops from 109 to
62 seconds. "

Link: http://lkml.kernel.org/r/20161125185518.29885-1-paul.burton@imgtec.com
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Tested-by: James Hartley <james.hartley@imgtec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 10 insertions, 1 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 05c0a59323bd..6da3169d3750 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5103,8 +5103,17 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
                 if (context != MEMMAP_EARLY)
                         goto not_early;
 
-                if (!early_pfn_valid(pfn))
+                if (!early_pfn_valid(pfn)) {
+#ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP
+                        /*
+                         * Skip to the pfn preceding the next valid one (or
+                         * end_pfn), such that we hit a valid pfn (or end_pfn)
+                         * on our next iteration of the loop.
+                         */
+                        pfn = memblock_next_valid_pfn(pfn, end_pfn) - 1;
+#endif
                         continue;
+                }
                 if (!early_pfn_in_nid(pfn, nid))
                         continue;
                 if (!update_defer_init(pgdat, pfn, end_pfn, &nr_initialised))