linux-kernel - Re: [PATCH v3 14/17] cxl: docs/allocation/page-allocator

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Message-ID: <6822b0ff24080_49706100ae@dwillia2-mobl4.notmuch>
Date: Mon, 12 May 2025 19:39:59 -0700
From: <dan.j.williams@...el.com>
To: Gregory Price <gourry@...rry.net>, Matthew Wilcox <willy@...radead.org>
CC: <linux-cxl@...r.kernel.org>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <kernel-team@...a.com>, <dave@...olabs.net>,
	<jonathan.cameron@...wei.com>, <dave.jiang@...el.com>,
	<alison.schofield@...el.com>, <vishal.l.verma@...el.com>,
	<ira.weiny@...el.com>, <dan.j.williams@...el.com>, <corbet@....net>
Subject: Re: [PATCH v3 14/17] cxl: docs/allocation/page-allocator

Gregory Price wrote:
> On Mon, May 12, 2025 at 06:52:31PM +0100, Matthew Wilcox wrote:
> > > 
> > > Feel free to submit patches that deletes the existing code if you want
> > > it removed from the documentation.
> > 
> > Who sneaked that in when?
> 
> The ACPI and EFI folks when they allowed for CXL memory to be marked 
> EFI_CONVENTIONAL_MEMORY - which means Linux can't actually differentiate
> between DRAM and CXL during __init and brings it online in the page
> allocator as SystemRAM in ZONE_NORMAL (attached to the NUMA node that
> maps to the Proximity Domain in the SRAT).
> 
> Not sure there's anything you can do about that.
> 
> And for DAX:
> 
> 09d09e04d2 (cxl/dax: Create dax devices for CXL RAM regions)
> 
> Which allows for EFI_MEMORY_SP / Soft Reserved CXL regions to be brought
> up as a DAX devices (which can be bound to SystemRAM via DAX kmem).
> 
> Wasn't much sneaking going on here - DAX kmem has been around and hacked
> on since 2019, and probably some years before that.

Right.

These interfaces have been there for a long time and this documentation
is simply catching up with what is there today. I called for all of this
documentation to go upstream and have no problem defending it to Linus.
Appreciate all the work here Gregory!

Now, is device-dax and dax_kmem the long term solution for exposing
memory of this relative performance class? After LSF/MM this year I am
convinced the answer is "no". Specifically I want to see a solution that
meets what this astute LWN commenter recommended:

https://lwn.net/Articles/1017142/

We can delete documentation and infrastructure once we have the
replacement interface upstream and can start a deprecation process.