[Patch v2] mm/hotplug: Only respect mem= parameter during boot stage

[Baoquan He]

In commit 357b4da50a62 ("x86: respect memory size limiting via mem=
parameter") a global varialbe global max_mem_size is added to store
the value parsed from 'mem= ', then checked when memory region is
added. This truly stops those DIMM from being added into system memory
during boot-time.

However, it also limits the later memory hotplug functionality. Any
memory board can't be hot added any more if its region is beyond the
max_mem_size. System will print error like below:

[  216.387164] acpi PNP0C80:02: add_memory failed
[  216.389301] acpi PNP0C80:02: acpi_memory_enable_device() error
[  216.392187] acpi PNP0C80:02: Enumeration failure

>From document of 'mem= ' parameter, it should be a restriction during
boot, but not impact the system memory adding/removing after booting.

  mem=nn[KMG]     [KNL,BOOT] Force usage of a specific amount of memory
	          ...

So fix it by also checking if it's during boot-time when restrict memory
adding. Otherwise, skip the restriction.

Fixes: 357b4da50a62 ("x86: respect memory size limiting via mem= parameter")
Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
---
 mm/memory_hotplug.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 55ac23ef11c1..989707295d15 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -105,7 +105,11 @@ static struct resource *register_memory_resource(u64 start, u64 size)
 	unsigned long flags =  IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY;
 	char *resource_name = "System RAM";
 
-	if (start + size > max_mem_size)
+	/*
+	 * Make sure value parsed from 'mem=' only restricts memory adding
+	 * during boot-time, so that memory hotplug won't be impacted.
+	 */
+	if (start + size > max_mem_size && system_state < SYSTEM_RUNNING)
 		return ERR_PTR(-E2BIG);
 
 	/*
-- 
2.17.2