[PATCH v2 2/3] x86: probe memblock size advisement value during mm init

[Gregory Price <gourry@gourry.net>]

Systems with hotplug may provide an advisement value on what the
memblock size should be.  Probe this value when the rest of the
configuration values are considered.

The new heuristic is as follows

1) set_memory_block_size_order value if already set (cmdline param)
2) minimum block size if memory is less than large block limit
3) [new] hotplug advise: lesser of advise value or memory alignment
4) Max block size if system is bare-metal
5) Largest size that aligns to end of memory.

Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Gregory Price <gourry@gourry.net>
---
 arch/x86/mm/init_64.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index ff253648706f..b72923b12d99 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1439,6 +1439,7 @@ static unsigned long probe_memory_block_size(void)
 {
 	unsigned long boot_mem_end = max_pfn << PAGE_SHIFT;
 	unsigned long bz;
+	int order;
 
 	/* If memory block size has been set, then use it */
 	bz = set_memory_block_size;
@@ -1451,6 +1452,21 @@ static unsigned long probe_memory_block_size(void)
 		goto done;
 	}
 
+	/* Consider hotplug advisement value (if set) */
+	order = memblock_probe_size_order();
+	bz = order > 0 ? (1UL << order) : 0;
+	if (bz) {
+		/* Align down to max and up to min supported */
+		bz = max(min(bz, MAX_BLOCK_SIZE), MIN_MEMORY_BLOCK_SIZE);
+		/* Use lesser of advisement and end of memory alignment */
+		for (; bz > MIN_MEMORY_BLOCK_SIZE; bz >>= 1) {
+			if (IS_ALIGNED(boot_mem_end, bz))
+				goto done;
+		}
+		/* Barring clean alignment, default to min block size */
+		goto done;
+	}
+
 	/*
 	 * Use max block size to minimize overhead on bare metal, where
 	 * alignment for memory hotplug isn't a concern.
-- 
2.43.0