linux-kernel - [PATCH v4 3/5] kdump, documentation: describe craskernel CMA reservation

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Message-ID: <aDoU9_xxl7SP7S_o@dwarf.suse.cz>
Date: Fri, 30 May 2025 22:28:39 +0200
From: Jiri Bohac <jbohac@...e.cz>
To: Baoquan He <bhe@...hat.com>, Vivek Goyal <vgoyal@...hat.com>,
	Dave Young <dyoung@...hat.com>, kexec@...ts.infradead.org
Cc: Philipp Rudo <prudo@...hat.com>, Donald Dutile <ddutile@...hat.com>,
	Pingfan Liu <piliu@...hat.com>, Tao Liu <ltao@...hat.com>,
	linux-kernel@...r.kernel.org,
	David Hildenbrand <dhildenb@...hat.com>,
	Michal Hocko <mhocko@...e.cz>
Subject: [PATCH v4 3/5] kdump, documentation: describe craskernel CMA
 reservation

Describe the new crashkernel ",cma" suffix in Documentation/

Signed-off-by: Jiri Bohac <jbohac@...e.cz>
---
 Documentation/admin-guide/kdump/kdump.rst     | 21 ++++++++++++++++++
 .../admin-guide/kernel-parameters.txt         | 22 +++++++++++++++++++
 2 files changed, 43 insertions(+)

diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
index 1f7f14c6e184..089665731509 100644
--- a/Documentation/admin-guide/kdump/kdump.rst
+++ b/Documentation/admin-guide/kdump/kdump.rst
@@ -311,6 +311,27 @@ crashkernel syntax
 
             crashkernel=0,low
 
+4) crashkernel=size,cma
+
+	Reserve additional crash kernel memory from CMA. This reservation is
+	usable by the first system's userspace memory and kernel movable
+	allocations (memory balloon, zswap). Pages allocated from this memory
+	range will not be included in the vmcore so this should not be used if
+	dumping of userspace memory is intended and it has to be expected that
+	some movable kernel pages may be missing from the dump.
+
+	A standard crashkernel reservation, as described above, is still needed
+	to hold the crash kernel and initrd.
+
+	This option increases the risk of a kdump failure: DMA transfers
+	configured by the first kernel may end up corrupting the second
+	kernel's memory.
+
+	This reservation method is intended for systems that can't afford to
+	sacrifice enough memory for standard crashkernel reservation and where
+	less reliable and possibly incomplete kdump is preferable to no kdump at
+	all.
+
 Boot into System Kernel
 -----------------------
 1) Update the boot loader (such as grub, yaboot, or lilo) configuration
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index ea81784be981..ee6be52dd8a5 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -983,6 +983,28 @@
 			0: to disable low allocation.
 			It will be ignored when crashkernel=X,high is not used
 			or memory reserved is below 4G.
+	crashkernel=size[KMG],cma
+			[KNL, X86] Reserve additional crash kernel memory from
+			CMA. This reservation is usable by the first system's
+			userspace memory and kernel movable allocations (memory
+			balloon, zswap). Pages allocated from this memory range
+			will not be included in the vmcore so this should not
+			be used if dumping of userspace memory is intended and
+			it has to be expected that some movable kernel pages
+			may be missing from the dump.
+
+			A standard crashkernel reservation, as described above,
+			is still needed to hold the crash kernel and initrd.
+
+			This option increases the risk of a kdump failure: DMA
+			transfers configured by the first kernel may end up
+			corrupting the second kernel's memory.
+
+			This reservation method is intended for systems that
+			can't afford to sacrifice enough memory for standard
+			crashkernel reservation and where less reliable and
+			possibly incomplete kdump is preferable to no kdump at
+			all.
 
 	cryptomgr.notests
 			[KNL] Disable crypto self-tests

-- 
Jiri Bohac <jbohac@...e.cz>
SUSE Labs, Prague, Czechia