Re: [RFC 1/4] mm: create N_COHERENT_MEMORY

[Reza Arbab <arbab@linux.vnet.ibm.com>]

On Wed, Apr 19, 2017 at 05:52:39PM +1000, Balbir Singh wrote:
>In this patch we create N_COHERENT_MEMORY, which is different
>from N_MEMORY. A node hotplugged as coherent memory will have
>this state set. The expectation then is that this memory gets
>onlined like regular nodes. Memory allocation from such nodes
>occurs only when the the node is contained explicitly in the
>mask.

Finally got around to test drive this. From what I can see, as expected,
both kernel and userspace seem to ignore these nodes, unless you 
allocate specifically from them. Very convenient.

Is "online_coherent"/MMOP_ONLINE_COHERENT the right way to trigger this?  
That mechanism is used to specify zone, and only for a single block of 
memory. This concept applies to the node as a whole. I think it should 
be independent of memory onlining.

I mean, let's say online_kernel N blocks, some of them get allocated, 
and then you online_coherent block N+1, flipping the entire node into 
N_COHERENT_MEMORY. That doesn't seem right.

That said, this set as it stands needs an adjustment when based on top 
of Michal's onlining revamp [1]. As-is, allow_online_pfn_range() is 
returning false. The patch below fixed it for me.

[1] http://lkml.kernel.org/r/20170421120512.23960-1-mhocko@kernel.org

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 4a535f1..ccb7a84 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -869,16 +869,20 @@ bool allow_online_pfn_range(int nid, unsigned long pfn, unsigned long nr_pages,
 	 * though so let's stick with it for simplicity for now.
 	 * TODO make sure we do not overlap with ZONE_DEVICE
 	 */
-	if (online_type == MMOP_ONLINE_KERNEL) {
+	switch (online_type) {
+	case MMOP_ONLINE_KERNEL:
 		if (zone_is_empty(movable_zone))
 			return true;
 		return movable_zone->zone_start_pfn >= pfn + nr_pages;
-	} else if (online_type == MMOP_ONLINE_MOVABLE) {
+	case MMOP_ONLINE_MOVABLE:
 		return zone_end_pfn(normal_zone) <= pfn;
+	case MMOP_ONLINE_KEEP:
+	case MMOP_ONLINE_COHERENT:
+		/* These will always succeed and inherit the current zone */
+		return true;
 	}
 
-	/* MMOP_ONLINE_KEEP will always succeed and inherits the current zone */
-	return online_type == MMOP_ONLINE_KEEP;
+	return false;
 }
 
 static void __meminit resize_zone_range(struct zone *zone, unsigned long start_pfn,


-- 
Reza Arbab

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