Re: [PATCH v5 2/3] arm64: mmu: avoid allocating pages while splitting the linear mapping

On Mon, Jan 05, 2026 at 08:23:27PM +0000, Yeoreum Yun wrote:

+static int __init linear_map_prealloc_split_pgtables(void)
+{
+	int ret, i;
+	unsigned long lstart = _PAGE_OFFSET(vabits_actual);
+	unsigned long lend = PAGE_END;
+	unsigned long kstart = (unsigned long)lm_alias(_stext);
+	unsigned long kend = (unsigned long)lm_alias(__init_begin);
+
+	const struct mm_walk_ops collect_to_split_ops = {
+		.pud_entry	= collect_to_split_pud_entry,
+		.pmd_entry	= collect_to_split_pmd_entry
+	};

Why do we need to rewalk the page-table here instead of collating the
number of block mappings we put down when creating the linear map in
the first place?

+	split_pgtables_idx = 0;
+	split_pgtables_count = 0;
+
+	ret = walk_kernel_page_table_range_lockless(lstart, kstart,
+						    &collect_to_split_ops,
+						    NULL, NULL);
+	if (!ret)
+		ret = walk_kernel_page_table_range_lockless(kend, lend,
+							    &collect_to_split_ops,
+							    NULL, NULL);
+	if (ret || !split_pgtables_count)
+		goto error;
+
+	ret = -ENOMEM;
+
+	split_pgtables = kvmalloc(split_pgtables_count * sizeof(struct ptdesc *),
+				  GFP_KERNEL | __GFP_ZERO);
+	if (!split_pgtables)
+		goto error;
+
+	for (i = 0; i < split_pgtables_count; i++) {
+		/* The page table will be filled during splitting, so zeroing it is unnecessary. */
+		split_pgtables[i] = pagetable_alloc(GFP_PGTABLE_KERNEL & ~__GFP_ZERO, 0);
+		if (!split_pgtables[i])
+			goto error;

This looks potentially expensive on the boot path and only gets worse as
the amount of memory grows. Maybe we should predicate this preallocation
on preempt-rt?

Will