powerpc/mm/radix: Split linear mapping on hot-unplug

This patch splits the linear mapping if the hot-unplug range is
smaller than the mapping size. The code detects if the mapping needs
to be split into a smaller size and if so, uses the stop machine
infrastructure to clear the existing mapping and then remap the
remaining range using a smaller page size.

The code will skip any region of the mapping that overlaps with kernel
text and warn about it once. We don't want to remove a mapping where
the kernel text and the LMB we intend to remove overlap in the same
TLB mapping as it may affect the currently executing code.

I've tested these changes under a kvm guest with 2 vcpus, from a split
mapping point of view, some of the caveats mentioned above applied to
the testing I did.

Fixes: 4b5d62ca17a1 ("powerpc/mm: add radix__remove_section_mapping()")
Signed-off-by: Balbir Singh <bsingharora@gmail.com>
[mpe: Tweak change log to match updated behaviour]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/pgtable-radix.c

@@ -17,6 +17,7 @@
 #include <linux/of_fdt.h>
 #include <linux/mm.h>
 #include <linux/string_helpers.h>
+#include <linux/stop_machine.h>
 
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
@@ -685,6 +686,30 @@ static void free_pmd_table(pmd_t *pmd_start, pud_t *pud)
 	pud_clear(pud);
 }
 
+struct change_mapping_params {
+	pte_t *pte;
+	unsigned long start;
+	unsigned long end;
+	unsigned long aligned_start;
+	unsigned long aligned_end;
+};
+
+static int stop_machine_change_mapping(void *data)
+{
+	struct change_mapping_params *params =
+			(struct change_mapping_params *)data;
+
+	if (!data)
+		return -1;
+
+	spin_unlock(&init_mm.page_table_lock);
+	pte_clear(&init_mm, params->aligned_start, params->pte);
+	create_physical_mapping(params->aligned_start, params->start);
+	create_physical_mapping(params->end, params->aligned_end);
+	spin_lock(&init_mm.page_table_lock);
+	return 0;
+}
+
 static void remove_pte_table(pte_t *pte_start, unsigned long addr,
 			     unsigned long end)
 {
@@ -713,6 +738,52 @@ static void remove_pte_table(pte_t *pte_start, unsigned long addr,
 	}
 }
 
+/*
+ * clear the pte and potentially split the mapping helper
+ */
+static void split_kernel_mapping(unsigned long addr, unsigned long end,
+				unsigned long size, pte_t *pte)
+{
+	unsigned long mask = ~(size - 1);
+	unsigned long aligned_start = addr & mask;
+	unsigned long aligned_end = addr + size;
+	struct change_mapping_params params;
+	bool split_region = false;
+
+	if ((end - addr) < size) {
+		/*
+		 * We're going to clear the PTE, but not flushed
+		 * the mapping, time to remap and flush. The
+		 * effects if visible outside the processor or
+		 * if we are running in code close to the
+		 * mapping we cleared, we are in trouble.
+		 */
+		if (overlaps_kernel_text(aligned_start, addr) ||
+			overlaps_kernel_text(end, aligned_end)) {
+			/*
+			 * Hack, just return, don't pte_clear
+			 */
+			WARN_ONCE(1, "Linear mapping %lx->%lx overlaps kernel "
+				  "text, not splitting\n", addr, end);
+			return;
+		}
+		split_region = true;
+	}
+
+	if (split_region) {
+		params.pte = pte;
+		params.start = addr;
+		params.end = end;
+		params.aligned_start = addr & ~(size - 1);
+		params.aligned_end = min_t(unsigned long, aligned_end,
+				(unsigned long)__va(memblock_end_of_DRAM()));
+		stop_machine(stop_machine_change_mapping, &params, NULL);
+		return;
+	}
+
+	pte_clear(&init_mm, addr, pte);
+}
+
 static void remove_pmd_table(pmd_t *pmd_start, unsigned long addr,
 			     unsigned long end)
 {
@@ -728,13 +799,7 @@ static void remove_pmd_table(pmd_t *pmd_start, unsigned long addr,
 			continue;
 
 		if (pmd_huge(*pmd)) {
-			if (!IS_ALIGNED(addr, PMD_SIZE) ||
-			    !IS_ALIGNED(next, PMD_SIZE)) {
-				WARN_ONCE(1, "%s: unaligned range\n", __func__);
-				continue;
-			}
-
-			pte_clear(&init_mm, addr, (pte_t *)pmd);
+			split_kernel_mapping(addr, end, PMD_SIZE, (pte_t *)pmd);
 			continue;
 		}
 
@@ -759,13 +824,7 @@ static void remove_pud_table(pud_t *pud_start, unsigned long addr,
 			continue;
 
 		if (pud_huge(*pud)) {
-			if (!IS_ALIGNED(addr, PUD_SIZE) ||
-			    !IS_ALIGNED(next, PUD_SIZE)) {
-				WARN_ONCE(1, "%s: unaligned range\n", __func__);
-				continue;
-			}
-
-			pte_clear(&init_mm, addr, (pte_t *)pud);
+			split_kernel_mapping(addr, end, PUD_SIZE, (pte_t *)pud);
 			continue;
 		}
 
@@ -791,13 +850,7 @@ static void remove_pagetable(unsigned long start, unsigned long end)
 			continue;
 
 		if (pgd_huge(*pgd)) {
-			if (!IS_ALIGNED(addr, PGDIR_SIZE) ||
-			    !IS_ALIGNED(next, PGDIR_SIZE)) {
-				WARN_ONCE(1, "%s: unaligned range\n", __func__);
-				continue;
-			}
-
-			pte_clear(&init_mm, addr, (pte_t *)pgd);
+			split_kernel_mapping(addr, end, PGDIR_SIZE, (pte_t *)pgd);
 			continue;
 		}