linux-kernel - Re: [PATCH] DAX: warn when kmem regions are truncated for memory block alignment.

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Message-ID: <Z-remBNWEej6KX3-@gourry-fedora-PF4VCD3F>
Date: Mon, 31 Mar 2025 14:27:36 -0400
From: Gregory Price <gourry@...rry.net>
To: dan.j.williams@...el.com
Cc: nvdimm@...ts.linux.dev, linux-kernel@...r.kernel.org,
	kernel-team@...a.com, dan.j.williams@...el.com,
	vishal.l.verma@...el.com, dave.jiang@...el.com,
	linux-cxl@...r.kernel.org, david@...hat.com
Subject: Re: [PATCH] DAX: warn when kmem regions are truncated for memory
 block alignment.

On Fri, Mar 21, 2025 at 02:07:31PM -0400, Gregory Price wrote:
> Device capacity intended for use as system ram should be aligned to the
> architecture-defined memory block size or that capacity will be silently
> truncated and capacity stranded.
> 
> As hotplug dax memory becomes more prevelant, the memory block size
> alignment becomes more important for platform and device vendors to
> pay attention to - so this truncation should not be silent.
> 
> This issue is particularly relevant for CXL Dynamic Capacity devices,
> whose capacity may arrive in spec-aligned but block-misaligned chunks.
> 
> Example:
>  [...] kmem dax0.0: dax region truncated 2684354560 bytes - alignment
>  [...] kmem dax1.0: dax region truncated 1610612736 bytes - alignment
> 
> Signed-off-by: Gregory Price <gourry@...rry.net>

Gentle pokes.  There were a couple questions last week whether we should
warn here or actually fix something in memory-hotplug.

Notes from CXL Boot to Bash session discussions:

We discussed [1] how this auto-sizing can cause 1GB huge page
allocation failures (assuming you online as ZONE_NORMAL). That means
ACPI-informed sizing by default would potentially be harmful to existing
systems and adding yet-another-boot-option just seems nasty.

I've since dropped acpi-informed block size patch[2].  If there are opinions
otherwise, I can continue pushing it.

We also discussed[3] variable-sized blocks having some nasty corner cases.
Not unsolvable, but doesn't help users in the short term.

There was some brief discussion about whether a hotplug memblock with a
portion as offline pages would be possible.  This seems hacky?  There
was another patch set discussing this, but I can't seem to find it.

I debated whether to warn here or in ACPI.  This seemed more accurate,
as platforms could simply over-reserve HPA space to avoid the issue.

Thoughts?
~Gregory

[1] https://lore.kernel.org/all/bda4cf52-d81a-4935-b45a-09e9439e33b6@redhat.com/
[2] https://lore.kernel.org/linux-mm/20250127153405.3379117-1-gourry@gourry.net/
[3]https://lore.kernel.org/all/b4b312c8-1117-45cd-a3c3-c8747aca51bd@redhat.com/