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

Web:        https://git.kernel.org/torvalds/c/4dd5f8a99e791a8c6500e3592f3ce81ae7edcde1
Commit:     4dd5f8a99e791a8c6500e3592f3ce81ae7edcde1
Parent:     eeb715c3e995fbdda0cc05e61216c6c5609bce66
Refname:    refs/heads/master
Author:     Balbir Singh <[email protected]>
AuthorDate: Wed Feb 7 17:35:51 2018 +1100
Committer:  Michael Ellerman <[email protected]>
CommitDate: Thu Feb 8 23:56:11 2018 +1100

    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 <[email protected]>
    [mpe: Tweak change log to match updated behaviour]
    Signed-off-by: Michael Ellerman <[email protected]>
---
 arch/powerpc/mm/pgtable-radix.c | 95 ++++++++++++++++++++++++++++++++---------
 1 file changed, 74 insertions(+), 21 deletions(-)

diff --git a/arch/powerpc/mm/pgtable-radix.c b/arch/powerpc/mm/pgtable-radix.c
index 96e07d1f673d..328ff9abc333 100644
--- a/arch/powerpc/mm/pgtable-radix.c
+++ b/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;
-			}