Subject: [PATCH v1 1/3] powerpc/mm/radix: Create separate mappings for hot-plugged memory

From: Bharata B Rao <[email-masked]>
Date: Tue, 23 Jun 2020 13:00:15 +0530

Memory that gets hot-plugged _during_ boot (and not the memory
that gets plugged in after boot), is mapped with 1G mappings
and will undergo splitting when it is unplugged. The splitting
code has a few issues:

1. Recursive locking
--------------------
Memory unplug path takes cpu_hotplug_lock and calls stop_machine()
for splitting the mappings. However stop_machine() takes
cpu_hotplug_lock again causing deadlock.

2. BUG: sleeping function called from in_atomic() context
---------------------------------------------------------
Memory unplug path (remove_pagetable) takes init_mm.page_table_lock
spinlock and later calls stop_machine() which does wait_for_completion()

3. Bad unlock unbalance
-----------------------
Memory unplug path takes init_mm.page_table_lock spinlock and calls
stop_machine(). The stop_machine thread function runs in a different
thread context (migration thread) which tries to release and reaquire
ptl. Releasing ptl from a different thread than which acquired it
causes bad unlock unbalance.

These problems can be avoided if we avoid mapping hot-plugged memory
with 1G mapping, thereby removing the need for splitting them during
unplug. Hence, during radix init, identify the hot-plugged memory region
and create separate mappings for each LMB so that they don't get mapped
with 1G mappings. The identification of hot-plugged memory has become
possible after the commit b6eca183e23e ("powerpc/kernel: Enables memory
hot-remove after reboot on pseries guests").

To create separate mappings for every LMB in the hot-plugged
region, we need lmb-size for which we use memory_block_size_bytes().
Since this is early init time code, the machine type isn't probed yet
and hence memory_block_size_bytes() would return the default LMB size
as 16MB. Hence we end up issuing more number of mapping requests
than earlier.

Signed-off-by: Bharata B Rao <[email-masked]>
---
 arch/powerpc/mm/book3s64/radix_pgtable.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/mm/book3s64/radix_pgtable.c b/arch/powerpc/mm/book3s64/radix_pgtable.c
index 8acb96de0e48..ffccfe00ca2a 100644
--- a/arch/powerpc/mm/book3s64/radix_pgtable.c
+++ b/arch/powerpc/mm/book3s64/radix_pgtable.c
@@ -16,6 +16,7 @@
 #include <linux/hugetlb.h>
 #include <linux/string_helpers.h>
 #include <linux/stop_machine.h>
+#include <linux/memory.h>
 
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
@@ -320,6 +321,8 @@ static void __init radix_init_pgtable(void)
 {
 	unsigned long rts_field;
 	struct memblock_region *reg;
+	phys_addr_t addr;
+	u64 lmb_size = memory_block_size_bytes();
 
 	/* We don't support slb for radix */
 	mmu_slb_size = 0;
@@ -338,9 +341,15 @@ static void __init radix_init_pgtable(void)
 			continue;
 		}
 
-		WARN_ON(create_physical_mapping(reg->base,
-						reg->base + reg->size,
-						-1, PAGE_KERNEL));
+		if (memblock_is_hotpluggable(reg)) {
+			for (addr = reg->base; addr < (reg->base + reg->size);
+			     addr += lmb_size)
+				WARN_ON(create_physical_mapping(addr,
+					addr + lmb_size, -1, PAGE_KERNEL));
+		} else
+			WARN_ON(create_physical_mapping(reg->base,
+							reg->base + reg->size,
+							-1, PAGE_KERNEL));
 	}
 
 	/* Find out how many PID bits are supported */
-- 
2.21.3