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From:   Rongwei Wang <rongwei.wang@linux.alibaba.com>
To:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Cc:     akpm@linux-foundation.org, willy@infradead.org,
        catalin.marinas@arm.com, dave.hansen@linux.intel.com,
        tj@kernel.org, mingo@redhat.com
Subject: [PATCH RFC 4/5] mm/numa: support CONFIG_NUMA_EMU for arm64
Date:   Thu, 12 Oct 2023 10:48:41 +0800
Message-Id: <20231012024842.99703-5-rongwei.wang@linux.alibaba.com>
In-Reply-To: <20231012024842.99703-1-rongwei.wang@linux.alibaba.com>
References: <20231012024842.99703-1-rongwei.wang@linux.alibaba.com>
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The CONFIG_NUMA_EMU migrates from x86/Kconfig
to mm/Kconfig. Now x86 and arm64 support it.

Signed-off-by: Rongwei Wang <rongwei.wang@linux.alibaba.com>
---
 arch/x86/Kconfig             |   8 -
 arch/x86/mm/Makefile         |   1 -
 arch/x86/mm/numa_emulation.c | 585 -----------------------------------
 drivers/base/arch_numa.c     |   3 +
 include/asm-generic/numa.h   |  12 +
 mm/Kconfig                   |   8 +
 mm/numa.c                    |  12 +
 7 files changed, 35 insertions(+), 594 deletions(-)
 delete mode 100644 arch/x86/mm/numa_emulation.c

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 66bfabae8814..13438bfe2ec1 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1568,14 +1568,6 @@ config X86_64_ACPI_NUMA
 	help
 	  Enable ACPI SRAT based node topology detection.
 
-config NUMA_EMU
-	bool "NUMA emulation"
-	depends on NUMA
-	help
-	  Enable NUMA emulation. A flat machine will be split
-	  into virtual nodes when booted with "numa=fake=N", where N is the
-	  number of nodes. This is only useful for debugging.
-
 config NODES_SHIFT
 	int "Maximum NUMA Nodes (as a power of 2)" if !MAXSMP
 	range 1 10
diff --git a/arch/x86/mm/Makefile b/arch/x86/mm/Makefile
index c80febc44cd2..1581f17e5de4 100644
--- a/arch/x86/mm/Makefile
+++ b/arch/x86/mm/Makefile
@@ -56,7 +56,6 @@ obj-$(CONFIG_MMIOTRACE_TEST)	+= testmmiotrace.o
 obj-$(CONFIG_NUMA)		+= numa.o numa_$(BITS).o
 obj-$(CONFIG_AMD_NUMA)		+= amdtopology.o
 obj-$(CONFIG_ACPI_NUMA)		+= srat.o
-obj-$(CONFIG_NUMA_EMU)		+= numa_emulation.o
 
 obj-$(CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS)	+= pkeys.o
 obj-$(CONFIG_RANDOMIZE_MEMORY)			+= kaslr.o
diff --git a/arch/x86/mm/numa_emulation.c b/arch/x86/mm/numa_emulation.c
deleted file mode 100644
index 9a9305367fdd..000000000000
--- a/arch/x86/mm/numa_emulation.c
+++ /dev/null
@@ -1,585 +0,0 @@
-// SPDX-License-Identifier: GPL-2.0
-/*
- * NUMA emulation
- */
-#include <linux/kernel.h>
-#include <linux/errno.h>
-#include <linux/topology.h>
-#include <linux/memblock.h>
-#include <asm/dma.h>
-
-#include "numa_internal.h"
-
-static int emu_nid_to_phys[MAX_NUMNODES];
-static char *emu_cmdline __initdata;
-
-int __init numa_emu_cmdline(char *str)
-{
-	emu_cmdline = str;
-	return 0;
-}
-
-static int __init emu_find_memblk_by_nid(int nid, const struct numa_meminfo *mi)
-{
-	int i;
-
-	for (i = 0; i < mi->nr_blks; i++)
-		if (mi->blk[i].nid == nid)
-			return i;
-	return -ENOENT;
-}
-
-static u64 __init mem_hole_size(u64 start, u64 end)
-{
-	unsigned long start_pfn = PFN_UP(start);
-	unsigned long end_pfn = PFN_DOWN(end);
-
-	if (start_pfn < end_pfn)
-		return PFN_PHYS(absent_pages_in_range(start_pfn, end_pfn));
-	return 0;
-}
-
-/*
- * Sets up nid to range from @start to @end.  The return value is -errno if
- * something went wrong, 0 otherwise.
- */
-static int __init emu_setup_memblk(struct numa_meminfo *ei,
-				   struct numa_meminfo *pi,
-				   int nid, int phys_blk, u64 size)
-{
-	struct numa_memblk *eb = &ei->blk[ei->nr_blks];
-	struct numa_memblk *pb = &pi->blk[phys_blk];
-
-	if (ei->nr_blks >= NR_NODE_MEMBLKS) {
-		pr_err("NUMA: Too many emulated memblks, failing emulation\n");
-		return -EINVAL;
-	}
-
-	ei->nr_blks++;
-	eb->start = pb->start;
-	eb->end = pb->start + size;
-	eb->nid = nid;
-
-	if (emu_nid_to_phys[nid] == NUMA_NO_NODE)
-		emu_nid_to_phys[nid] = pb->nid;
-
-	pb->start += size;
-	if (pb->start >= pb->end) {
-		WARN_ON_ONCE(pb->start > pb->end);
-		numa_remove_memblk_from(phys_blk, pi);
-	}
-
-	printk(KERN_INFO "Faking node %d at [mem %#018Lx-%#018Lx] (%LuMB)\n",
-	       nid, eb->start, eb->end - 1, (eb->end - eb->start) >> 20);
-	return 0;
-}
-
-/*
- * Sets up nr_nodes fake nodes interleaved over physical nodes ranging from addr
- * to max_addr.
- *
- * Returns zero on success or negative on error.
- */
-static int __init split_nodes_interleave(struct numa_meminfo *ei,
-					 struct numa_meminfo *pi,
-					 u64 addr, u64 max_addr, int nr_nodes)
-{
-	nodemask_t physnode_mask = numa_nodes_parsed;
-	u64 size;
-	int big;
-	int nid = 0;
-	int i, ret;
-
-	if (nr_nodes <= 0)
-		return -1;
-	if (nr_nodes > MAX_NUMNODES) {
-		pr_info("numa=fake=%d too large, reducing to %d\n",
-			nr_nodes, MAX_NUMNODES);
-		nr_nodes = MAX_NUMNODES;
-	}
-
-	/*
-	 * Calculate target node size.  x86_32 freaks on __udivdi3() so do
-	 * the division in ulong number of pages and convert back.
-	 */
-	size = max_addr - addr - mem_hole_size(addr, max_addr);
-	size = PFN_PHYS((unsigned long)(size >> PAGE_SHIFT) / nr_nodes);
-
-	/*
-	 * Calculate the number of big nodes that can be allocated as a result
-	 * of consolidating the remainder.
-	 */
-	big = ((size & ~FAKE_NODE_MIN_HASH_MASK) * nr_nodes) /
-		FAKE_NODE_MIN_SIZE;
-
-	size &= FAKE_NODE_MIN_HASH_MASK;
-	if (!size) {
-		pr_err("Not enough memory for each node.  "
-			"NUMA emulation disabled.\n");
-		return -1;
-	}
-
-	/*
-	 * Continue to fill physical nodes with fake nodes until there is no
-	 * memory left on any of them.
-	 */
-	while (!nodes_empty(physnode_mask)) {
-		for_each_node_mask(i, physnode_mask) {
-			u64 dma32_end = PFN_PHYS(MAX_DMA32_PFN);
-			u64 start, limit, end;
-			int phys_blk;
-
-			phys_blk = emu_find_memblk_by_nid(i, pi);
-			if (phys_blk < 0) {
-				node_clear(i, physnode_mask);
-				continue;
-			}
-			start = pi->blk[phys_blk].start;
-			limit = pi->blk[phys_blk].end;
-			end = start + size;
-
-			if (nid < big)
-				end += FAKE_NODE_MIN_SIZE;
-
-			/*
-			 * Continue to add memory to this fake node if its
-			 * non-reserved memory is less than the per-node size.
-			 */
-			while (end - start - mem_hole_size(start, end) < size) {
-				end += FAKE_NODE_MIN_SIZE;
-				if (end > limit) {
-					end = limit;
-					break;
-				}
-			}
-
-			/*
-			 * If there won't be at least FAKE_NODE_MIN_SIZE of
-			 * non-reserved memory in ZONE_DMA32 for the next node,
-			 * this one must extend to the boundary.
-			 */
-			if (end < dma32_end && dma32_end - end -
-			    mem_hole_size(end, dma32_end) < FAKE_NODE_MIN_SIZE)
-				end = dma32_end;
-
-			/*
-			 * If there won't be enough non-reserved memory for the
-			 * next node, this one must extend to the end of the
-			 * physical node.
-			 */
-			if (limit - end - mem_hole_size(end, limit) < size)
-				end = limit;
-
-			ret = emu_setup_memblk(ei, pi, nid++ % nr_nodes,
-					       phys_blk,
-					       min(end, limit) - start);
-			if (ret < 0)
-				return ret;
-		}
-	}
-	return 0;
-}
-
-/*
- * Returns the end address of a node so that there is at least `size' amount of
- * non-reserved memory or `max_addr' is reached.
- */
-static u64 __init find_end_of_node(u64 start, u64 max_addr, u64 size)
-{
-	u64 end = start + size;
-
-	while (end - start - mem_hole_size(start, end) < size) {
-		end += FAKE_NODE_MIN_SIZE;
-		if (end > max_addr) {
-			end = max_addr;
-			break;
-		}
-	}
-	return end;
-}
-
-static u64 uniform_size(u64 max_addr, u64 base, u64 hole, int nr_nodes)
-{
-	unsigned long max_pfn = PHYS_PFN(max_addr);
-	unsigned long base_pfn = PHYS_PFN(base);
-	unsigned long hole_pfns = PHYS_PFN(hole);
-
-	return PFN_PHYS((max_pfn - base_pfn - hole_pfns) / nr_nodes);
-}
-
-/*
- * Sets up fake nodes of `size' interleaved over physical nodes ranging from
- * `addr' to `max_addr'.
- *
- * Returns zero on success or negative on error.
- */
-static int __init split_nodes_size_interleave_uniform(struct numa_meminfo *ei,
-					      struct numa_meminfo *pi,
-					      u64 addr, u64 max_addr, u64 size,
-					      int nr_nodes, struct numa_memblk *pblk,
-					      int nid)
-{
-	nodemask_t physnode_mask = numa_nodes_parsed;
-	int i, ret, uniform = 0;
-	u64 min_size;
-
-	if ((!size && !nr_nodes) || (nr_nodes && !pblk))
-		return -1;
-
-	/*
-	 * In the 'uniform' case split the passed in physical node by
-	 * nr_nodes, in the non-uniform case, ignore the passed in
-	 * physical block and try to create nodes of at least size
-	 * @size.
-	 *
-	 * In the uniform case, split the nodes strictly by physical
-	 * capacity, i.e. ignore holes. In the non-uniform case account
-	 * for holes and treat @size as a minimum floor.
-	 */
-	if (!nr_nodes)
-		nr_nodes = MAX_NUMNODES;
-	else {
-		nodes_clear(physnode_mask);
-		node_set(pblk->nid, physnode_mask);
-		uniform = 1;
-	}
-
-	if (uniform) {
-		min_size = uniform_size(max_addr, addr, 0, nr_nodes);
-		size = min_size;
-	} else {
-		/*
-		 * The limit on emulated nodes is MAX_NUMNODES, so the
-		 * size per node is increased accordingly if the
-		 * requested size is too small.  This creates a uniform
-		 * distribution of node sizes across the entire machine
-		 * (but not necessarily over physical nodes).
-		 */
-		min_size = uniform_size(max_addr, addr,
-				mem_hole_size(addr, max_addr), nr_nodes);
-	}
-	min_size = ALIGN(max(min_size, FAKE_NODE_MIN_SIZE), FAKE_NODE_MIN_SIZE);
-	if (size < min_size) {
-		pr_err("Fake node size %LuMB too small, increasing to %LuMB\n",
-			size >> 20, min_size >> 20);
-		size = min_size;
-	}
-	size = ALIGN_DOWN(size, FAKE_NODE_MIN_SIZE);
-
-	/*
-	 * Fill physical nodes with fake nodes of size until there is no memory
-	 * left on any of them.
-	 */
-	while (!nodes_empty(physnode_mask)) {
-		for_each_node_mask(i, physnode_mask) {
-			u64 dma32_end = PFN_PHYS(MAX_DMA32_PFN);
-			u64 start, limit, end;
-			int phys_blk;
-
-			phys_blk = emu_find_memblk_by_nid(i, pi);
-			if (phys_blk < 0) {
-				node_clear(i, physnode_mask);
-				continue;
-			}
-
-			start = pi->blk[phys_blk].start;
-			limit = pi->blk[phys_blk].end;
-
-			if (uniform)
-				end = start + size;
-			else
-				end = find_end_of_node(start, limit, size);
-			/*
-			 * If there won't be at least FAKE_NODE_MIN_SIZE of
-			 * non-reserved memory in ZONE_DMA32 for the next node,
-			 * this one must extend to the boundary.
-			 */
-			if (end < dma32_end && dma32_end - end -
-			    mem_hole_size(end, dma32_end) < FAKE_NODE_MIN_SIZE)
-				end = dma32_end;
-
-			/*
-			 * If there won't be enough non-reserved memory for the
-			 * next node, this one must extend to the end of the
-			 * physical node.
-			 */
-			if ((limit - end - mem_hole_size(end, limit) < size)
-					&& !uniform)
-				end = limit;
-
-			ret = emu_setup_memblk(ei, pi, nid++ % MAX_NUMNODES,
-					       phys_blk,
-					       min(end, limit) - start);
-			if (ret < 0)
-				return ret;
-		}
-	}
-	return nid;
-}
-
-static int __init split_nodes_size_interleave(struct numa_meminfo *ei,
-					      struct numa_meminfo *pi,
-					      u64 addr, u64 max_addr, u64 size)
-{
-	return split_nodes_size_interleave_uniform(ei, pi, addr, max_addr, size,
-			0, NULL, 0);
-}
-
-static int __init setup_emu2phys_nid(int *dfl_phys_nid)
-{
-	int i, max_emu_nid = 0;
-
-	*dfl_phys_nid = NUMA_NO_NODE;
-	for (i = 0; i < ARRAY_SIZE(emu_nid_to_phys); i++) {
-		if (emu_nid_to_phys[i] != NUMA_NO_NODE) {
-			max_emu_nid = i;
-			if (*dfl_phys_nid == NUMA_NO_NODE)
-				*dfl_phys_nid = emu_nid_to_phys[i];
-		}
-	}
-
-	return max_emu_nid;
-}
-
-/**
- * numa_emulation - Emulate NUMA nodes
- * @numa_meminfo: NUMA configuration to massage
- * @numa_dist_cnt: The size of the physical NUMA distance table
- *
- * Emulate NUMA nodes according to the numa=fake kernel parameter.
- * @numa_meminfo contains the physical memory configuration and is modified
- * to reflect the emulated configuration on success.  @numa_dist_cnt is
- * used to determine the size of the physical distance table.
- *
- * On success, the following modifications are made.
- *
- * - @numa_meminfo is updated to reflect the emulated nodes.
- *
- * - __apicid_to_node[] is updated such that APIC IDs are mapped to the
- *   emulated nodes.
- *
- * - NUMA distance table is rebuilt to represent distances between emulated
- *   nodes.  The distances are determined considering how emulated nodes
- *   are mapped to physical nodes and match the actual distances.
- *
- * - emu_nid_to_phys[] reflects how emulated nodes are mapped to physical
- *   nodes.  This is used by numa_add_cpu() and numa_remove_cpu().
- *
- * If emulation is not enabled or fails, emu_nid_to_phys[] is filled with
- * identity mapping and no other modification is made.
- */
-void __init numa_emulation(struct numa_meminfo *numa_meminfo, int numa_dist_cnt)
-{
-	static struct numa_meminfo ei __initdata;
-	static struct numa_meminfo pi __initdata;
-	const u64 max_addr = PFN_PHYS(max_pfn);
-	u8 *phys_dist = NULL;
-	size_t phys_size = numa_dist_cnt * numa_dist_cnt * sizeof(phys_dist[0]);
-	int max_emu_nid, dfl_phys_nid;
-	int i, j, ret;
-
-	if (!emu_cmdline)
-		goto no_emu;
-
-	memset(&ei, 0, sizeof(ei));
-	pi = *numa_meminfo;
-
-	for (i = 0; i < MAX_NUMNODES; i++)
-		emu_nid_to_phys[i] = NUMA_NO_NODE;
-
-	/*
-	 * If the numa=fake command-line contains a 'M' or 'G', it represents
-	 * the fixed node size.  Otherwise, if it is just a single number N,
-	 * split the system RAM into N fake nodes.
-	 */
-	if (strchr(emu_cmdline, 'U')) {
-		nodemask_t physnode_mask = numa_nodes_parsed;
-		unsigned long n;
-		int nid = 0;
-
-		n = simple_strtoul(emu_cmdline, &emu_cmdline, 0);
-		ret = -1;
-		for_each_node_mask(i, physnode_mask) {
-			/*
-			 * The reason we pass in blk[0] is due to
-			 * numa_remove_memblk_from() called by
-			 * emu_setup_memblk() will delete entry 0
-			 * and then move everything else up in the pi.blk
-			 * array. Therefore we should always be looking
-			 * at blk[0].
-			 */
-			ret = split_nodes_size_interleave_uniform(&ei, &pi,
-					pi.blk[0].start, pi.blk[0].end, 0,
-					n, &pi.blk[0], nid);
-			if (ret < 0)
-				break;
-			if (ret < n) {
-				pr_info("%s: phys: %d only got %d of %ld nodes, failing\n",
-						__func__, i, ret, n);
-				ret = -1;
-				break;
-			}
-			nid = ret;
-		}
-	} else if (strchr(emu_cmdline, 'M') || strchr(emu_cmdline, 'G')) {
-		u64 size;
-
-		size = memparse(emu_cmdline, &emu_cmdline);
-		ret = split_nodes_size_interleave(&ei, &pi, 0, max_addr, size);
-	} else {
-		unsigned long n;
-
-		n = simple_strtoul(emu_cmdline, &emu_cmdline, 0);
-		ret = split_nodes_interleave(&ei, &pi, 0, max_addr, n);
-	}
-	if (*emu_cmdline == ':')
-		emu_cmdline++;
-
-	if (ret < 0)
-		goto no_emu;
-
-	if (numa_cleanup_meminfo(&ei) < 0) {
-		pr_warn("NUMA: Warning: constructed meminfo invalid, disabling emulation\n");
-		goto no_emu;
-	}
-
-	/* copy the physical distance table */
-	if (numa_dist_cnt) {
-		u64 phys;
-
-		phys = memblock_phys_alloc_range(phys_size, PAGE_SIZE, 0,
-						 PFN_PHYS(max_pfn_mapped));
-		if (!phys) {
-			pr_warn("NUMA: Warning: can't allocate copy of distance table, disabling emulation\n");
-			goto no_emu;
-		}
-		phys_dist = __va(phys);
-
-		for (i = 0; i < numa_dist_cnt; i++)
-			for (j = 0; j < numa_dist_cnt; j++)
-				phys_dist[i * numa_dist_cnt + j] =
-					node_distance(i, j);
-	}
-
-	/*
-	 * Determine the max emulated nid and the default phys nid to use
-	 * for unmapped nodes.
-	 */
-	max_emu_nid = setup_emu2phys_nid(&dfl_phys_nid);
-
-	/* commit */
-	*numa_meminfo = ei;
-
-	/* Make sure numa_nodes_parsed only contains emulated nodes */
-	nodes_clear(numa_nodes_parsed);
-	for (i = 0; i < ARRAY_SIZE(ei.blk); i++)
-		if (ei.blk[i].start != ei.blk[i].end &&
-		    ei.blk[i].nid != NUMA_NO_NODE)
-			node_set(ei.blk[i].nid, numa_nodes_parsed);
-
-	/*
-	 * Transform __apicid_to_node table to use emulated nids by
-	 * reverse-mapping phys_nid.  The maps should always exist but fall
-	 * back to zero just in case.
-	 */
-	for (i = 0; i < ARRAY_SIZE(__apicid_to_node); i++) {
-		if (__apicid_to_node[i] == NUMA_NO_NODE)
-			continue;
-		for (j = 0; j < ARRAY_SIZE(emu_nid_to_phys); j++)
-			if (__apicid_to_node[i] == emu_nid_to_phys[j])
-				break;
-		__apicid_to_node[i] = j < ARRAY_SIZE(emu_nid_to_phys) ? j : 0;
-	}
-
-	/* make sure all emulated nodes are mapped to a physical node */
-	for (i = 0; i < ARRAY_SIZE(emu_nid_to_phys); i++)
-		if (emu_nid_to_phys[i] == NUMA_NO_NODE)
-			emu_nid_to_phys[i] = dfl_phys_nid;
-
-	/* transform distance table */
-	numa_reset_distance();
-	for (i = 0; i < max_emu_nid + 1; i++) {
-		for (j = 0; j < max_emu_nid + 1; j++) {
-			int physi = emu_nid_to_phys[i];
-			int physj = emu_nid_to_phys[j];
-			int dist;
-
-			if (get_option(&emu_cmdline, &dist) == 2)
-				;
-			else if (physi >= numa_dist_cnt || physj >= numa_dist_cnt)
-				dist = physi == physj ?
-					LOCAL_DISTANCE : REMOTE_DISTANCE;
-			else
-				dist = phys_dist[physi * numa_dist_cnt + physj];
-
-			numa_set_distance(i, j, dist);
-		}
-	}
-
-	/* free the copied physical distance table */
-	memblock_free(phys_dist, phys_size);
-	return;
-
-no_emu:
-	/* No emulation.  Build identity emu_nid_to_phys[] for numa_add_cpu() */
-	for (i = 0; i < ARRAY_SIZE(emu_nid_to_phys); i++)
-		emu_nid_to_phys[i] = i;
-}
-
-#ifndef CONFIG_DEBUG_PER_CPU_MAPS
-void numa_add_cpu(int cpu)
-{
-	int physnid, nid;
-
-	nid = early_cpu_to_node(cpu);
-	BUG_ON(nid == NUMA_NO_NODE || !node_online(nid));
-
-	physnid = emu_nid_to_phys[nid];
-
-	/*
-	 * Map the cpu to each emulated node that is allocated on the physical
-	 * node of the cpu's apic id.
-	 */
-	for_each_online_node(nid)
-		if (emu_nid_to_phys[nid] == physnid)
-			cpumask_set_cpu(cpu, node_to_cpumask_map[nid]);
-}
-
-void numa_remove_cpu(int cpu)
-{
-	int i;
-
-	for_each_online_node(i)
-		cpumask_clear_cpu(cpu, node_to_cpumask_map[i]);
-}
-#else	/* !CONFIG_DEBUG_PER_CPU_MAPS */
-static void numa_set_cpumask(int cpu, bool enable)
-{
-	int nid, physnid;
-
-	nid = early_cpu_to_node(cpu);
-	if (nid == NUMA_NO_NODE) {
-		/* early_cpu_to_node() already emits a warning and trace */
-		return;
-	}
-
-	physnid = emu_nid_to_phys[nid];
-
-	for_each_online_node(nid) {
-		if (emu_nid_to_phys[nid] != physnid)
-			continue;
-
-		debug_cpumask_set_cpu(cpu, nid, enable);
-	}
-}
-
-void numa_add_cpu(int cpu)
-{
-	numa_set_cpumask(cpu, true);
-}
-
-void numa_remove_cpu(int cpu)
-{
-	numa_set_cpumask(cpu, false);
-}
-#endif	/* !CONFIG_DEBUG_PER_CPU_MAPS */
diff --git a/drivers/base/arch_numa.c b/drivers/base/arch_numa.c
index 5df0ad5cb09d..67bdbcd0caf9 100644
--- a/drivers/base/arch_numa.c
+++ b/drivers/base/arch_numa.c
@@ -13,6 +13,7 @@
 #include <linux/module.h>
 #include <linux/of.h>
 
+#include <asm-generic/numa.h>
 #include <asm/sections.h>
 
 struct pglist_data *node_data[MAX_NUMNODES] __read_mostly;
@@ -30,6 +31,8 @@ static __init int numa_parse_early_param(char *opt)
 		return -EINVAL;
 	if (str_has_prefix(opt, "off"))
 		numa_off = true;
+	if (!strncmp(opt, "fake=", 5))
+		return numa_emu_cmdline(opt + 5);
 
 	return 0;
 }
diff --git a/include/asm-generic/numa.h b/include/asm-generic/numa.h
index 929d7c582a73..4658155a070a 100644
--- a/include/asm-generic/numa.h
+++ b/include/asm-generic/numa.h
@@ -50,12 +50,24 @@ struct numa_meminfo {
 	struct numa_memblk	blk[NR_NODE_MEMBLKS];
 };
 
+#ifdef CONFIG_NUMA_EMU
+#define FAKE_NODE_MIN_SIZE	((u64)32 << 20)
+#define FAKE_NODE_MIN_HASH_MASK	(~(FAKE_NODE_MIN_SIZE - 1UL))
+
 extern struct numa_meminfo numa_meminfo;
+extern char *emu_cmdline __initdata;
 
+int numa_emu_cmdline(char *str);
 int __init numa_register_memblks(struct numa_meminfo *mi);
 int __init numa_cleanup_meminfo(struct numa_meminfo *mi);
 void __init numa_emulation(struct numa_meminfo *numa_meminfo,
 			   int numa_dist_cnt);
+#else
+static inline int numa_emu_cmdline(char *str)
+{
+	return -EINVAL;
+}
+#endif
 
 #else	/* CONFIG_NUMA */
 
diff --git a/mm/Kconfig b/mm/Kconfig
index 264a2df5ecf5..22bead675ee6 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -549,6 +549,14 @@ config ARCH_ENABLE_MEMORY_HOTPLUG
 config ARCH_ENABLE_MEMORY_HOTREMOVE
 	bool
 
+config NUMA_EMU
+	bool "NUMA emulation (EXPERIMENTAL)"
+	depends on NUMA && (X86 || ARM64)
+	help
+	  Enable NUMA emulation. A flat machine will be split
+	  into virtual nodes when booted with "numa=fake=N", where N is the
+	  number of nodes. This is only useful for debugging.
+
 # eventually, we can have this option just 'select SPARSEMEM'
 menuconfig MEMORY_HOTPLUG
 	bool "Memory hotplug"
diff --git a/mm/numa.c b/mm/numa.c
index 88277e8404f0..3cc01f06a2a6 100644
--- a/mm/numa.c
+++ b/mm/numa.c
@@ -16,6 +16,10 @@
 struct numa_meminfo numa_meminfo __initdata_or_meminfo;
 struct numa_meminfo numa_reserved_meminfo __initdata_or_meminfo;
 
+#ifdef CONFIG_NUMA_EMU
+char *emu_cmdline __initdata;
+#endif
+
 /*
  * Set nodes, which have memory in @mi, in *@nodemask.
  */
@@ -296,3 +300,11 @@ int __weak __init numa_register_memblks(struct numa_meminfo *mi)
 
 	return 0;
 }
+
+#ifdef CONFIG_NUMA_EMU
+int __init numa_emu_cmdline(char *str)
+{
+	emu_cmdline = str;
+	return 0;
+}
+#endif
-- 
2.32.0.3.gf3a3e56d6