arm64: mm: Optimize range_split_to_ptes()

Enter lazy_mmu mode while splitting a range of memory to pte mappings.
This causes barriers, which would otherwise be emitted after every pte
(and pmd/pud) write, to be deferred until exiting lazy_mmu mode.

For large systems, this is expected to significantly speed up fallback
to pte-mapping the linear map for the case where the boot CPU has
BBML2_NOABORT, but secondary CPUs do not. I haven't directly measured
it, but this is equivalent to commit 1fcb7ce ("arm64: mm: Batch dsb
and isb when populating pgtables").

Note that for the path from arch_kfence_init_pool(), we may sleep while
allocating memory inside the lazy_mmu mode. Sleeping is not allowed by
generic code inside lazy_mmu, but we know that the arm64 implementation
is sleep-safe. So this is ok and follows the same pattern already used
by split_kernel_leaf_mapping().

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: Yang Shi <yang@os.amperecomputing.com>
Signed-off-by: Will Deacon <will@kernel.org>

Author: ryanhrob

arch/arm64/mm/mmu.c

@@ -832,8 +832,14 @@ static const struct mm_walk_ops split_to_ptes_ops = {
 
 static int range_split_to_ptes(unsigned long start, unsigned long end, gfp_t gfp)
 {
-	return walk_kernel_page_table_range_lockless(start, end,
+	int ret;
+
+	arch_enter_lazy_mmu_mode();
+	ret = walk_kernel_page_table_range_lockless(start, end,
 					&split_to_ptes_ops, NULL, &gfp);
+	arch_leave_lazy_mmu_mode();
+
+	return ret;
 }
 
 static bool linear_map_requires_bbml2 __initdata;