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Message-ID: <697acf78acf70_3095100c@dwillia2-mobl4.notmuch>
Date: Wed, 28 Jan 2026 19:09:44 -0800
From: <dan.j.williams@...el.com>
To: <dan.j.williams@...el.com>, Smita Koralahalli
<Smita.KoralahalliChannabasappa@....com>, <linux-cxl@...r.kernel.org>,
<linux-kernel@...r.kernel.org>, <nvdimm@...ts.linux.dev>,
<linux-fsdevel@...r.kernel.org>, <linux-pm@...r.kernel.org>
CC: Ard Biesheuvel <ardb@...nel.org>, 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>,
Jonathan Cameron <jonathan.cameron@...wei.com>, Yazen Ghannam
<yazen.ghannam@....com>, Dave Jiang <dave.jiang@...el.com>, Davidlohr Bueso
<dave@...olabs.net>, 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>, Yao Xingtao <yaoxt.fnst@...itsu.com>,
Peter Zijlstra <peterz@...radead.org>, Greg Kroah-Hartman
<gregkh@...uxfoundation.org>, Nathan Fontenot <nathan.fontenot@....com>,
Terry Bowman <terry.bowman@....com>, Robert Richter <rrichter@....com>,
Benjamin Cheatham <benjamin.cheatham@....com>, Zhijian Li
<lizhijian@...itsu.com>, Borislav Petkov <bp@...en8.de>, Smita Koralahalli
<Smita.KoralahalliChannabasappa@....com>, Tomasz Wolski
<tomasz.wolski@...itsu.com>
Subject: Re: [PATCH v5 6/7] dax/hmem, cxl: Defer and resolve ownership of Soft
Reserved memory ranges
dan.j.williams@ wrote:
> Smita Koralahalli wrote:
> > The current probe time ownership check for Soft Reserved memory based
> > solely on CXL window intersection is insufficient. dax_hmem probing is not
> > always guaranteed to run after CXL enumeration and region assembly, which
> > can lead to incorrect ownership decisions before the CXL stack has
> > finished publishing windows and assembling committed regions.
> >
> > Introduce deferred ownership handling for Soft Reserved ranges that
> > intersect CXL windows at probe time by scheduling deferred work from
> > dax_hmem and waiting for the CXL stack to complete enumeration and region
> > assembly before deciding ownership.
> >
> > Evaluate ownership of Soft Reserved ranges based on CXL region
> > containment.
> >
> > - If all Soft Reserved ranges are fully contained within committed CXL
> > regions, DROP handling Soft Reserved ranges from dax_hmem and allow
> > dax_cxl to bind.
> >
> > - If any Soft Reserved range is not fully claimed by committed CXL
> > region, tear down all CXL regions and REGISTER the Soft Reserved
> > ranges with dax_hmem instead.
> >
> > While ownership resolution is pending, gate dax_cxl probing to avoid
> > binding prematurely.
> >
> > This enforces a strict ownership. Either CXL fully claims the Soft
> > Reserved ranges or it relinquishes it entirely.
> >
> > Co-developed-by: Dan Williams <dan.j.williams@...el.com>
> > Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> > Signed-off-by: Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>
> > ---
> > drivers/cxl/core/region.c | 25 ++++++++++++
> > drivers/cxl/cxl.h | 2 +
> > drivers/dax/cxl.c | 9 +++++
> > drivers/dax/hmem/hmem.c | 81 ++++++++++++++++++++++++++++++++++++++-
> > 4 files changed, 115 insertions(+), 2 deletions(-)
> >
[..]
> > diff --git a/drivers/dax/cxl.c b/drivers/dax/cxl.c
> > index 13cd94d32ff7..b7e90d6dd888 100644
> > --- a/drivers/dax/cxl.c
> > +++ b/drivers/dax/cxl.c
> > @@ -14,6 +14,15 @@ static int cxl_dax_region_probe(struct device *dev)
> > struct dax_region *dax_region;
> > struct dev_dax_data data;
> >
> > + switch (dax_cxl_mode) {
> > + case DAX_CXL_MODE_DEFER:
> > + return -EPROBE_DEFER;
>
> So, I think this causes a mess because now you have 2 workqueues (driver
> core defer-queue and hmem work) competing to disposition this device.
> What this seems to want is to only run in the post "soft reserve
> dispositioned" world. Something like (untested!)
>
> diff --git a/drivers/dax/cxl.c b/drivers/dax/cxl.c
> index 13cd94d32ff7..1162495eb317 100644
> --- a/drivers/dax/cxl.c
> +++ b/drivers/dax/cxl.c
> @@ -14,6 +14,9 @@ static int cxl_dax_region_probe(struct device *dev)
> struct dax_region *dax_region;
> struct dev_dax_data data;
>
> + /* Make sure that dax_cxl_mode is stable, only runs once at boot */
> + flush_hmem_work();
> +
It occurs to me that this likely insta-hangs because
wait_for_device_probe() waits forever for itself to flush. So it may
need to be a scheme where the cxl_dax_region_driver registration does
something like this (untested!):
diff --git a/drivers/dax/cxl.c b/drivers/dax/cxl.c
index 13cd94d32ff7..6a1a38b4f64b 100644
--- a/drivers/dax/cxl.c
+++ b/drivers/dax/cxl.c
@@ -41,7 +41,32 @@ static struct cxl_driver cxl_dax_region_driver = {
},
};
-module_cxl_driver(cxl_dax_region_driver);
+static void cxl_dax_region_driver_register(struct work_struct *work)
+{
+ flush_hmem_work();
+ cxl_driver_register(&cxl_dax_region_driver);
+}
+
+static DECLARE_WORK(cxl_dax_region_driver_work, cxl_dax_region_driver_register);
+
+static int __init cxl_dax_region_init(void)
+{
+ /*
+ * Need to resolve a race with dax_hmem wanting to drive regions
+ * instead of CXL
+ */
+ queue_work(system_long_wq, &cxl_dax_region_driver_work);
+ return 0;
+}
+module_init(cxl_dax_region_init);
+
+static void __exit cxl_dax_region_exit(void)
+{
+ flush_work(&cxl_dax_region_driver_work);
+ cxl_driver_unregister(&cxl_dax_region_driver);
+}
+module_exit(cxl_dax_region_exit);
+
MODULE_ALIAS_CXL(CXL_DEVICE_DAX_REGION);
MODULE_DESCRIPTION("CXL DAX: direct access to CXL regions");
MODULE_LICENSE("GPL");
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