linux-kernel - [PATCH 4/4] kdump, documentation: describe craskernel CMA reservation

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Message-ID: <ZWEAYdO7VfT02EJz@dwarf.suse.cz>
Date:   Fri, 24 Nov 2023 20:58:25 +0100
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:     linux-kernel@...r.kernel.org, mhocko@...e.cz
Subject: [PATCH 4/4] kdump, documentation: describe craskernel CMA reservation

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

---
 Documentation/admin-guide/kdump/kdump.rst       | 10 ++++++++++
 Documentation/admin-guide/kernel-parameters.txt |  7 +++++++
 2 files changed, 17 insertions(+)

diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
index 5762e7477a0c..4ec08e5843dc 100644
--- a/Documentation/admin-guide/kdump/kdump.rst
+++ b/Documentation/admin-guide/kdump/kdump.rst
@@ -317,6 +317,16 @@ crashkernel syntax
 
             crashkernel=0,low
 
+4) crashkernel=size,cma
+
+   Reserves additional memory from CMA. A standard crashkernel reservation, as
+   described above, is still needed, but can be just small enough to hold the
+   kernel and initrd. All the memory the crash kernel needs for its runtime and
+   for running the kdump userspace processes can be provided by this CMA
+   reservation, re-using memory available to the production system's userspace.
+   Because of this re-using, the CMA reservation should not be used if it's
+   intended to dump userspce memory.
+
 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 65731b060e3f..ee9fc40a97fd 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -914,6 +914,13 @@
 			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 1st system's userspace,
+			so this should not be used if dumping of userspace 
+			memory is intended. A standard crashkernel reservation, 
+			as described above, is still needed to hold the crash 
+			kernel and initrd. 
 
 	cryptomgr.notests
 			[KNL] Disable crypto self-tests

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