linux-kernel - Re: [PATCH v3 0/5] dax/hmem, cxl: Coordinate Soft Reserved handling with CXL

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Message-ID: <aOlxcaTUowFddiEQ@aschofie-mobl2.lan>
Date: Fri, 10 Oct 2025 13:49:53 -0700
From: Alison Schofield <alison.schofield@...el.com>
To: Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>
CC: <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<nvdimm@...ts.linux.dev>, <linux-fsdevel@...r.kernel.org>,
	<linux-pm@...r.kernel.org>, Davidlohr Bueso <dave@...olabs.net>, "Jonathan
 Cameron" <jonathan.cameron@...wei.com>, Dave Jiang <dave.jiang@...el.com>,
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	"Dan Williams" <dan.j.williams@...el.com>, Matthew Wilcox
	<willy@...radead.org>, Jan Kara <jack@...e.cz>, "Rafael J . Wysocki"
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	Zhijian Li <lizhijian@...itsu.com>, "Borislav Petkov" <bp@...en8.de>, Ard
 Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v3 0/5] dax/hmem, cxl: Coordinate Soft Reserved handling
 with CXL

On Mon, Oct 06, 2025 at 06:16:24PM -0700, Alison Schofield wrote:
> On Tue, Sep 30, 2025 at 04:47:52AM +0000, Smita Koralahalli wrote:
> > This series aims to address long-standing conflicts between dax_hmem and
> > CXL when handling Soft Reserved memory ranges.
> 
> Hi Smita,
> 
> Thanks for the updates Smita!
> 
> About those "long-standing conflicts": In the next rev, can you resurrect,
> or recreate the issues list that this set is addressing. It's been a
> long and winding road with several handoffs (me included) and it'll help
> keep the focus.
> 
> Hotplug works :)  Auto region comes up, we tear it down and can recreate it,
> in place, because the soft reserved resource is gone (no longer occupying
> the CXL Window and causing recreate to fail.)
> 
> !CONFIG_CXL_REGION works :) All resources go directly to DAX.
> 
> The scenario that is failing is handoff to DAX after region assembly
> failure. (Dan reminded me to check that today.) That is mostly related
> to Patch4, so I'll respond there.
> 
> --Alison

Hi Smita -

(after off-list chat w Smita about what is and is not included)

This CXL failover to DAX case is not implemented. In my response in Patch 4,
I cobbled something together that made it work in one test case. But to be
clear, there was some trickery in the CXL region driver to even do that.

One path forward is to update this set restating the issues it addresses, and
remove any code and comments that are tied to failing over to DAX after a
region assembly failure.

That leaves the issue Dan raised, "shutdown CXL in favor of vanilla DAX devices
as an emergency fallback for platform configuration quirks and bugs"[1], for a
future patch.

-- Alison

[1] The failover to DAX was last described in response to v5 of the 'prior' patchset.
https://lore.kernel.org/linux-cxl/20250715180407.47426-1-Smita.KoralahalliChannabasappa@amd.com/
https://lore.kernel.org/linux-cxl/687ffcc0ee1c8_137e6b100ed@dwillia2-xfh.jf.intel.com.notmuch/
https://lore.kernel.org/linux-cxl/68808fb4e4cbf_137e6b100cc@dwillia2-xfh.jf.intel.com.notmuch/

> 
>