powerpc/mm/radix: Remove split_kernel_mapping()

We split the page table mapping on memory unplug if the
linear range was mapped with huge page mapping (for ex: 1G)
The page table 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. The kernel always make sure the minimum unplug request is
SUBSECTION_SIZE for device memory and SECTION_SIZE for regular memory.

In preparation for such a change remove page table splitting support.

This essentially is a revert of
commit 4dd5f8a9 ("powerpc/mm/radix: Split linear mapping on hot-unplug")
Signed-off-by: Bharata B Rao <bharata@linux.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200709131925.922266-4-aneesh.kumar@linux.ibm.com

arch/powerpc/mm/book3s64/radix_pgtable.c

@@ -15,7 +15,6 @@
 #include <linux/mm.h>
 #include <linux/hugetlb.h>
 #include <linux/string_helpers.h>
-#include <linux/stop_machine.h>
 
 #include <asm/pgalloc.h>
 #include <asm/mmu_context.h>
@@ -722,32 +721,6 @@ static void free_pud_table(pud_t *pud_start, p4d_t *p4d)
 	p4d_clear(p4d);
 }
 
-struct change_mapping_params {
-	pte_t *pte;
-	unsigned long start;
-	unsigned long end;
-	unsigned long aligned_start;
-	unsigned long aligned_end;
-};
-
-static int __meminit 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(__pa(params->aligned_start),
-				__pa(params->start), -1, PAGE_KERNEL);
-	create_physical_mapping(__pa(params->end), __pa(params->aligned_end),
-				-1, PAGE_KERNEL);
-	spin_lock(&init_mm.page_table_lock);
-	return 0;
-}
-
 static void remove_pte_table(pte_t *pte_start, unsigned long addr,
 			     unsigned long end)
 {
@@ -776,52 +749,6 @@ static void remove_pte_table(pte_t *pte_start, unsigned long addr,
 	}
 }
 
-/*
- * clear the pte and potentially split the mapping helper
- */
-static void __meminit 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)
 {
@@ -837,7 +764,12 @@ static void remove_pmd_table(pmd_t *pmd_start, unsigned long addr,
 			continue;
 
 		if (pmd_is_leaf(*pmd)) {
-			split_kernel_mapping(addr, end, PMD_SIZE, (pte_t *)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);
 			continue;
 		}
 
@@ -862,7 +794,12 @@ static void remove_pud_table(pud_t *pud_start, unsigned long addr,
 			continue;
 
 		if (pud_is_leaf(*pud)) {
-			split_kernel_mapping(addr, end, PUD_SIZE, (pte_t *)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);
 			continue;
 		}
 
@@ -890,7 +827,13 @@ static void __meminit remove_pagetable(unsigned long start, unsigned long end)
 			continue;
 
 		if (p4d_is_leaf(*p4d)) {
-			split_kernel_mapping(addr, end, P4D_SIZE, (pte_t *)p4d);
+			if (!IS_ALIGNED(addr, P4D_SIZE) ||
+			    !IS_ALIGNED(next, P4D_SIZE)) {
+				WARN_ONCE(1, "%s: unaligned range\n", __func__);
+				continue;
+			}
+
+			pte_clear(&init_mm, addr, (pte_t *)pgd);
 			continue;
 		}