linux-kernel - Re: [PATCH] Documentation/driver-api/cxl: remove page-allocator quirk section

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Message-ID: <20251028160133.000004ca@huawei.com>
Date: Tue, 28 Oct 2025 16:01:33 +0000
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: Gregory Price <gourry@...rry.net>
CC: <linux-cxl@...r.kernel.org>, <dave@...olabs.net>, <dave.jiang@...el.com>,
	<alison.schofield@...el.com>, <vishal.l.verma@...el.com>,
	<ira.weiny@...el.com>, <dan.j.williams@...el.com>, <corbet@....net>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Documentation/driver-api/cxl: remove page-allocator
 quirk section

On Fri,  3 Oct 2025 10:32:32 -0400
Gregory Price <gourry@...rry.net> wrote:

> The node/zone quirk section of the cxl documentation is incorrect.
> The actual reason for fallback allocation misbehavior in the
> described configuration is due to a kswapd/reclaim thrashing scenario
> fixed by the linked patch.  Remove this section.
> 
> Link: https://lore.kernel.org/linux-mm/20250919162134.1098208-1-hannes@cmpxchg.org/
> Signed-off-by: Gregory Price <gourry@...rry.net>
Not looked into it in detail but I'll take your word for it that
the text below is wrong.

Reviewed-by: Jonathan Cameron <jonathan.cameron@...wei.com>

> ---
>  .../cxl/allocation/page-allocator.rst         | 31 -------------------
>  1 file changed, 31 deletions(-)
> 
> diff --git a/Documentation/driver-api/cxl/allocation/page-allocator.rst b/Documentation/driver-api/cxl/allocation/page-allocator.rst
> index 7b8fe1b8d5bb..3fa584a248bd 100644
> --- a/Documentation/driver-api/cxl/allocation/page-allocator.rst
> +++ b/Documentation/driver-api/cxl/allocation/page-allocator.rst
> @@ -41,37 +41,6 @@ To simplify this, the page allocator will prefer :code:`ZONE_MOVABLE` over
>  will fallback to allocate from :code:`ZONE_NORMAL`.
>  
>  
> -Zone and Node Quirks
> -====================
> -Let's consider a configuration where the local DRAM capacity is largely onlined
> -into :code:`ZONE_NORMAL`, with no :code:`ZONE_MOVABLE` capacity present. The
> -CXL capacity has the opposite configuration - all onlined in
> -:code:`ZONE_MOVABLE`.
> -
> -Under the default allocation policy, the page allocator will completely skip
> -:code:`ZONE_MOVABLE` as a valid allocation target.  This is because, as of
> -Linux v6.15, the page allocator does (approximately) the following: ::
> -
> -  for (each zone in local_node):
> -
> -    for (each node in fallback_order):
> -
> -      attempt_allocation(gfp_flags);
> -
> -Because the local node does not have :code:`ZONE_MOVABLE`, the CXL node is
> -functionally unreachable for direct allocation.  As a result, the only way
> -for CXL capacity to be used is via `demotion` in the reclaim path.
> -
> -This configuration also means that if the DRAM ndoe has :code:`ZONE_MOVABLE`
> -capacity - when that capacity is depleted, the page allocator will actually
> -prefer CXL :code:`ZONE_MOVABLE` pages over DRAM :code:`ZONE_NORMAL` pages.
> -
> -We may wish to invert this priority in future Linux versions.
> -
> -If `demotion` and `swap` are disabled, Linux will begin to cause OOM crashes
> -when the DRAM nodes are depleted. See the reclaim section for more details.
> -
> -
>  CGroups and CPUSets
>  ===================
>  Finally, assuming CXL memory is reachable via the page allocation (i.e. onlined