linux-kernel - Re: [RFC PATCH 6/6] cxl/region, dax/hmem: Guard CXL DAX region creation and tighten HMEM deps

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Message-ID: <2397ebb5-ae63-402e-bc23-339c74be9210@fujitsu.com>
Date: Mon, 1 Sep 2025 06:21:09 +0000
From: "Zhijian Li (Fujitsu)" <lizhijian@...itsu.com>
To: Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>,
	"linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
CC: Davidlohr Bueso <dave@...olabs.net>, Jonathan Cameron
	<jonathan.cameron@...wei.com>, Dave Jiang <dave.jiang@...el.com>, Alison
 Schofield <alison.schofield@...el.com>, Vishal Verma
	<vishal.l.verma@...el.com>, Ira Weiny <ira.weiny@...el.com>, Dan Williams
	<dan.j.williams@...el.com>, Matthew Wilcox <willy@...radead.org>, Jan Kara
	<jack@...e.cz>, "Rafael J . Wysocki" <rafael@...nel.org>, Len Brown
	<len.brown@...el.com>, Pavel Machek <pavel@...nel.org>, Li Ming
	<ming.li@...omail.com>, Jeff Johnson <jeff.johnson@....qualcomm.com>, Ying
 Huang <huang.ying.caritas@...il.com>, "Xingtao Yao (Fujitsu)"
	<yaoxt.fnst@...itsu.com>, Peter Zijlstra <peterz@...radead.org>, Greg KH
	<gregkh@...uxfoundation.org>, Nathan Fontenot <nathan.fontenot@....com>,
	Terry Bowman <terry.bowman@....com>, Robert Richter <rrichter@....com>,
	Benjamin Cheatham <benjamin.cheatham@....com>, PradeepVineshReddy Kodamati
	<PradeepVineshReddy.Kodamati@....com>
Subject: Re: [RFC PATCH 6/6] cxl/region, dax/hmem: Guard CXL DAX region
 creation and tighten HMEM deps



On 22/08/2025 11:42, Smita Koralahalli wrote:
> Prevent cxl_region_probe() from unconditionally calling into
> devm_cxl_add_dax_region() when the DEV_DAX_CXL driver is not enabled.
> Wrap the call with IS_ENABLED(CONFIG_DEV_DAX_CXL) so region probe skips
> DAX setup cleanly if no consumer is present.

A question came to mind:
  
Why is the case of `CXL_REGION && !DEV_DAX_CXL` necessary? It appears to fall back to the hmem driver in that scenario.
If so, could we instead simplify it as follows?
  
--- a/drivers/cxl/Kconfig
+++ b/drivers/cxl/Kconfig
@@ -200,6 +200,7 @@ config CXL_REGION
         depends on SPARSEMEM
         select MEMREGION
         select GET_FREE_REGION
+       select DEV_DAX_CXL

> 
> In parallel, update DEV_DAX_HMEM’s Kconfig to depend on
> !CXL_BUS || (CXL_ACPI && CXL_PCI) || m. This ensures:
> 
> Built-in (y) HMEM is allowed when CXL is disabled, or when the full
> CXL discovery stack is built-in. Module (m) HMEM remains always possible.

Hmm,IIUC, `dax_hmem` isn't exclusively designed for CXL. It could support other special memory types (e.g., HBM).

Thanks
Zhijian



> 
> Signed-off-by: Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>
> ---
> I did not want to override Dan’s original approach, so I am posting this
> as an RFC.
> 
> This patch addresses a corner case when applied on top of Patches 1–5.
> 
> When DEV_DAX_HMEM=y and CXL=m, the DEV_DAX_CXL option ends up disabled.
> In that configuration, with Patches 1–5 applied, ownership of the Soft
> Reserved ranges falls back to dax_hmem. As a result, /proc/iomem looks
> like this:
> 
> 850000000-284fffffff : CXL Window 0
>    850000000-284fffffff : region3
>      850000000-284fffffff : Soft Reserved
>        850000000-284fffffff : dax0.0
>          850000000-284fffffff : System RAM (kmem)
> 2850000000-484fffffff : CXL Window 1
>    2850000000-484fffffff : region4
>      2850000000-484fffffff : Soft Reserved
>        2850000000-484fffffff : dax1.0
>          2850000000-484fffffff : System RAM (kmem)
> 4850000000-684fffffff : CXL Window 2
>    4850000000-684fffffff : region5
>      4850000000-684fffffff : Soft Reserved
>        4850000000-684fffffff : dax2.0
>          4850000000-684fffffff : System RAM (kmem)
> 
> In this case the dax devices are created by dax_hmem, not by dax_cxl.
> Consequently, a "cxl disable-region <regionx>" operation does not
> unregister these devices. In addition, the dmesg output can be misleading
> to users, since it looks like the CXL region driver created the devdax
> devices:
> 
>    devm_cxl_add_region: cxl_acpi ACPI0017:00: decoder0.2: created region5
>    ..
>    ..
> 
> This patch addresses those situations. I am not entirely sure how clean
> the approach of using “|| m” is, so I am sending it as RFC for feedback.
> ---
>   drivers/cxl/core/region.c | 4 +++-
>   drivers/dax/Kconfig       | 1 +
>   2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 71cc42d05248..6a2c21e55dbc 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -3617,7 +3617,9 @@ static int cxl_region_probe(struct device *dev)
>   					p->res->start, p->res->end, cxlr,
>   					is_system_ram) > 0)
>   			return 0;
> -		return devm_cxl_add_dax_region(cxlr);
> +		if (IS_ENABLED(CONFIG_DEV_DAX_CXL))
> +			return devm_cxl_add_dax_region(cxlr);
> +		return 0;
>   	default:
>   		dev_dbg(&cxlr->dev, "unsupported region mode: %d\n",
>   			cxlr->mode);
> diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig
> index 3683bb3f2311..fd12cca91c78 100644
> --- a/drivers/dax/Kconfig
> +++ b/drivers/dax/Kconfig
> @@ -30,6 +30,7 @@ config DEV_DAX_PMEM
>   config DEV_DAX_HMEM
>   	tristate "HMEM DAX: direct access to 'specific purpose' memory"
>   	depends on EFI_SOFT_RESERVE
> +	depends on !CXL_BUS || (CXL_ACPI && CXL_PCI) || m
>   	select NUMA_KEEP_MEMINFO if NUMA_MEMBLKS
>   	default DEV_DAX
>   	help