mm: skip invalid pages block at a time in zero_resv_unresv()

The role of zero_resv_unavail() is to make sure that every struct page
that is allocated but is not backed by memory that is accessible by
kernel is zeroed and not in some uninitialized state.

Since struct pages are allocated in blocks (2M pages in x86 case), we
can skip pageblock_nr_pages at a time, when the first one is found to be
invalid.

This optimization may help since now on x86 every hole in e820 maps is
marked as reserved in memblock, and thus will go through this function.

This function is called before sched_clock() is initialized, so I used
my x86 early boot clock patches to measure the performance improvement.

With 1T hole on i7-8700 currently we would take 0.606918s of boot time,
but with this optimization 0.001103s.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Pavel Tatashin <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Reviewed-by: Naoya Horiguchi <[email protected]>
Cc: Pasha Tatashin <[email protected]>
Cc: Steven Sistare <[email protected]>
Cc: Daniel Jordan <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: "Huang, Ying" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 4 insertions, 1 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 0922ef5d2e46..33f6745bb649 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6405,8 +6405,11 @@ void __paginginit zero_resv_unavail(void)
 	pgcnt = 0;
 	for_each_resv_unavail_range(i, &start, &end) {
 		for (pfn = PFN_DOWN(start); pfn < PFN_UP(end); pfn++) {
-			if (!pfn_valid(ALIGN_DOWN(pfn, pageblock_nr_pages)))
+			if (!pfn_valid(ALIGN_DOWN(pfn, pageblock_nr_pages))) {
+				pfn = ALIGN_DOWN(pfn, pageblock_nr_pages)
+					+ pageblock_nr_pages - 1;
 				continue;
+			}
 			mm_zero_struct_page(pfn_to_page(pfn));
 			pgcnt++;
 		}