[PATCH part1 v7 4/6] x86/mem-hotplug: Support initialize page tablesin bottom-up

[Zhang Yanfei]

From: Tang Chen <tangchen@xxxxxxxxxxxxxx>

The Linux kernel cannot migrate pages used by the kernel. As a
result, kernel pages cannot be hot-removed. So we cannot allocate
hotpluggable memory for the kernel.

In a memory hotplug system, any numa node the kernel resides in
should be unhotpluggable. And for a modern server, each node could
have at least 16GB memory. So memory around the kernel image is
highly likely unhotpluggable.

ACPI SRAT (System Resource Affinity Table) contains the memory
hotplug info. But before SRAT is parsed, memblock has already
started to allocate memory for the kernel. So we need to prevent
memblock from doing this.

So direct memory mapping page tables setup is the case. init_mem_mapping()
is called before SRAT is parsed. To prevent page tables being allocated
within hotpluggable memory, we will use bottom-up direction to allocate
page tables from the end of kernel image to the higher memory.

Note:
As for allocating page tables in lower memory, TJ said:
[This is an optional behavior which is triggered by a very specific
kernel boot param, which I suspect is gonna need to stick around to
support memory hotplug in the current setup unless we add another
layer of address translation to support memory hotplug.]

As for page tables may occupy too much lower memory if using 4K mapping
(CONFIG_DEBUG_PAGEALLOC and CONFIG_KMEMCHECK both disable using >4k
pages), TJ said:
[But as I said in the same paragraph, parsing SRAT earlier doesn't
solve the problem in itself either. Ignoring the option if 4k mapping
is required and memory consumption would be prohibitive should work, no?
Something like that would be necessary if we're gonna worry about cases
like this no matter how we implement it, but, frankly, I'm not sure this
is something worth worrying about.]

Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Acked-by: Toshi Kani <toshi.kani@xxxxxx>
Signed-off-by: Tang Chen <tangchen@xxxxxxxxxxxxxx>
Signed-off-by: Zhang Yanfei <zhangyanfei@xxxxxxxxxxxxxx>
---
 arch/x86/mm/init.c |   66 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 64 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index ea2be79..b6892a7 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -458,6 +458,51 @@ static void __init memory_map_top_down(unsigned long map_start,
 		init_range_memory_mapping(real_end, map_end);
 }
 
+/**
+ * memory_map_bottom_up - Map [map_start, map_end) bottom up
+ * @map_start: start address of the target memory range
+ * @map_end: end address of the target memory range
+ *
+ * This function will setup direct mapping for memory range
+ * [map_start, map_end) in bottom-up. Since we have limited the
+ * bottom-up allocation above the kernel, the page tables will
+ * be allocated just above the kernel and we map the memory
+ * in [map_start, map_end) in bottom-up.
+ */
+static void __init memory_map_bottom_up(unsigned long map_start,
+					unsigned long map_end)
+{
+	unsigned long next, new_mapped_ram_size, start;
+	unsigned long mapped_ram_size = 0;
+	/* step_size need to be small so pgt_buf from BRK could cover it */
+	unsigned long step_size = PMD_SIZE;
+
+	start = map_start;
+	min_pfn_mapped = start >> PAGE_SHIFT;
+
+	/*
+	 * We start from the bottom (@map_start) and go to the top (@map_end).
+	 * The memblock_find_in_range() gets us a block of RAM from the
+	 * end of RAM in [min_pfn_mapped, max_pfn_mapped) used as new pages
+	 * for page table.
+	 */
+	while (start < map_end) {
+		if (map_end - start > step_size) {
+			next = round_up(start + 1, step_size);
+			if (next > map_end)
+				next = map_end;
+		} else
+			next = map_end;
+
+		new_mapped_ram_size = init_range_memory_mapping(start, next);
+		start = next;
+
+		if (new_mapped_ram_size > mapped_ram_size)
+			step_size <<= STEP_SIZE_SHIFT;
+		mapped_ram_size += new_mapped_ram_size;
+	}
+}
+
 void __init init_mem_mapping(void)
 {
 	unsigned long end;
@@ -473,8 +518,25 @@ void __init init_mem_mapping(void)
 	/* the ISA range is always mapped regardless of memory holes */
 	init_memory_mapping(0, ISA_END_ADDRESS);
 
-	/* setup direct mapping for range [ISA_END_ADDRESS, end) in top-down*/
-	memory_map_top_down(ISA_END_ADDRESS, end);
+	/*
+	 * If the allocation is in bottom-up direction, we setup direct mapping
+	 * in bottom-up, otherwise we setup direct mapping in top-down.
+	 */
+	if (memblock_bottom_up()) {
+		unsigned long kernel_end = __pa_symbol(_end);
+
+		/*
+		 * we need two separate calls here. This is because we want to
+		 * allocate page tables above the kernel. So we first map
+		 * [kernel_end, end) to make memory above the kernel be mapped
+		 * as soon as possible. And then use page tables allocated above
+		 * the kernel to map [ISA_END_ADDRESS, kernel_end).
+		 */
+		memory_map_bottom_up(kernel_end, end);
+		memory_map_bottom_up(ISA_END_ADDRESS, kernel_end);
+	} else {
+		memory_map_top_down(ISA_END_ADDRESS, end);
+	}
 
 #ifdef CONFIG_X86_64
 	if (max_pfn > max_low_pfn) {
-- 
1.7.1

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