Re: [PATCH v6 07/10] dax: plumb hotplug online_type through dax

[Gregory Price <gourry@gourry.net>]

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On Fri, Jul 10, 2026 at 05:44:58PM -0700, Dan Williams (nvidia) wrote:
> Gregory Price wrote:
> > On Thu, Jul 09, 2026 at 06:08:45PM -0400, Gregory Price wrote:
> > > On Thu, Jul 09, 2026 at 02:46:39PM -0700, Dan Williams (nvidia) wrote:
> > > 
> > > This was more a matter of having the DEFAULT set consistently across
> > > the dax driver variant probe() functions to make the behavior explicit.
> > > I didn't want an un-set value bug to creep in here somehow.
> > > 
> > > Happy to drop them if you think that's unneeded.
> > > 
> > 
> > Ah
> > 
> > Not setting the value in each of those places is equivalent to setting
> > MMOP_OFFLINE (0), so better to just set DEFAULT regardless.
> > 
> > So unless you have strong feelings i will keep them as-is.
> 
> Right, the mild feelings are only coming from the changelog mismatch
> which says "Oh no, device-dax drivers can not specify their online type
> besides the default" and all this series does is keep the status quo.
> 
> That can be had by just having unconditional:
> 
>      online_type = mhp_get_default_online_type();
> 
> ...in dev_dax_kmem_probe() and get rid of dev_dax->online_type until the
> first user arrives. If you are respinning the series and that patch
> drops, yay. If not, oh well.

if you prefer, i can add the CXL build option to this series instead of
the follow up (attached).  The code is fully contained in DAX anyway,
and I have it sitting off in another branch anyway.

I suppose with this, we can terminate the entire series and the first
user that grows an opinion can add the following:

cxl/cxl.h:
    struct cxl_dax_region *cxlr_dax {
    +   int online_type
    };

dax/cxl.c:
   data.online_type = (cxlr_dax.online_type == DAX_ONLINE_DEFAULT) ?
                      cxl_dax_online_type() : cxlr_dax.online_type;

~Gregory

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From b2cb5c4f5f03dd698df97ae3db2c0207db4acd83 Mon Sep 17 00:00:00 2001
From: Gregory Price <gourry@gourry.net>
Date: Thu, 9 Jul 2026 19:31:28 -0700
Subject: [PATCH] dax/cxl: add build-time CXL RAM region auto-online policy

CXL devices using kmem follow the single global auto-online policy.

On systems with multiple drivers using memory hotplug, this makes
management more complex - a single special kmem device forces all
kmem devices to be managed manually in userland.

Add a build-time option to select CXL device online policy.

  - System default   -> DAX_ONLINE_DEFAULT (default)
  - Online           -> MMOP_ONLINE
  - Online (movable) -> MMOP_ONLINE_MOVABLE
  - Online (kernel)  -> MMOP_ONLINE_KERNEL
  - Unplugged        -> DAX_KMEM_UNPLUGGED

Default to system default to retain userland backward compatibility.

Signed-off-by: Gregory Price <gourry@gourry.net>
---
 drivers/cxl/Kconfig | 41 +++++++++++++++++++++++++++++++++++++++++
 drivers/dax/cxl.c   | 15 ++++++++++++++-
 2 files changed, 55 insertions(+), 1 deletion(-)

diff --git a/drivers/cxl/Kconfig b/drivers/cxl/Kconfig
index 80aeb0d556bd7..52450225dbb11 100644
--- a/drivers/cxl/Kconfig
+++ b/drivers/cxl/Kconfig
@@ -212,6 +212,47 @@ config CXL_REGION
 
 	  If unsure say 'y'
 
+choice
+	prompt "CXL Memory Hotplug Policy"
+	depends on CXL_REGION
+	default CXL_MEMHP_POLICY_SYSTEM_DEFAULT
+	help
+	  Select the online policy the CXL driver requests when a region is
+	  hotplugged as System RAM.
+
+	  If unsure, select "System default".
+
+config CXL_MEMHP_POLICY_SYSTEM_DEFAULT
+	bool "System default"
+	help
+	  Follow the system-wide memory hotplug policy.
+
+config CXL_MEMHP_POLICY_ONLINE
+	bool "Online"
+	help
+	  Automatically online memory and let the kernel choose the zone.
+
+config CXL_MEMHP_POLICY_ONLINE_MOVABLE
+	bool "Online (movable)"
+	help
+	  Automatically online the memory into ZONE_MOVABLE. Choose this for
+	  memory that may need to be hotremoved later, or if you do not want
+	  unmovable, kernel allocations to land on this memory.
+
+config CXL_MEMHP_POLICY_ONLINE_KERNEL
+	bool "Online (kernel)"
+	help
+	  Automatically online the memory into ZONE_NORMAL. The kernel may
+	  place unmovable allocations on this memory.
+
+config CXL_MEMHP_POLICY_UNPLUGGED
+	bool "Unplugged"
+	help
+	  Create the dax/kmem device but add no memory at bind time.
+	  Choose this if a userland orchestrator will manage the device.
+
+endchoice
+
 config CXL_REGION_INVALIDATION_TEST
 	bool "CXL: Region Cache Management Bypass (TEST)"
 	depends on CXL_REGION
diff --git a/drivers/dax/cxl.c b/drivers/dax/cxl.c
index 1a7ec62122134..15f70d578c22b 100644
--- a/drivers/dax/cxl.c
+++ b/drivers/dax/cxl.c
@@ -6,6 +6,19 @@
 #include "../cxl/cxl.h"
 #include "bus.h"
 
+static int cxl_dax_online_type(void)
+{
+	if (IS_ENABLED(CONFIG_CXL_MEMHP_POLICY_UNPLUGGED))
+		return DAX_KMEM_UNPLUGGED;
+	if (IS_ENABLED(CONFIG_CXL_MEMHP_POLICY_ONLINE))
+		return MMOP_ONLINE;
+	if (IS_ENABLED(CONFIG_CXL_MEMHP_POLICY_ONLINE_MOVABLE))
+		return MMOP_ONLINE_MOVABLE;
+	if (IS_ENABLED(CONFIG_CXL_MEMHP_POLICY_ONLINE_KERNEL))
+		return MMOP_ONLINE_KERNEL;
+	return DAX_ONLINE_DEFAULT;
+}
+
 static int cxl_dax_region_probe(struct device *dev)
 {
 	struct cxl_dax_region *cxlr_dax = to_cxl_dax_region(dev);
@@ -27,7 +40,7 @@ static int cxl_dax_region_probe(struct device *dev)
 		.id = -1,
 		.size = range_len(&cxlr_dax->hpa_range),
 		.memmap_on_memory = true,
-		.online_type = DAX_ONLINE_DEFAULT,
+		.online_type = cxl_dax_online_type(),
 	};
 
 	return PTR_ERR_OR_ZERO(devm_create_dev_dax(&data));
-- 
2.53.0-Meta