linux-kernel - [PATCH v2 3/5] kdump, documentation: describe craskernel CMA reservation

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Message-ID: <Z7deQKy1F8BW1ibi@dwarf.suse.cz>
Date: Thu, 20 Feb 2025 17:54:24 +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: 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 v2 3/5] kdump, documentation: describe craskernel CMA
 reservation

Describe the new crashkernel ",cma" suffix in Documentation/
---
 Documentation/admin-guide/kdump/kdump.rst     | 19 +++++++++++++++++--
 .../admin-guide/kernel-parameters.txt         | 14 ++++++++++++++
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
index 5376890adbeb..3f7ff301c604 100644
--- a/Documentation/admin-guide/kdump/kdump.rst
+++ b/Documentation/admin-guide/kdump/kdump.rst
@@ -213,8 +213,7 @@ Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
    kernel.
 
    Otherwise it should be the start of memory region reserved for
-   second kernel using boot parameter "crashkernel=Y@X". Here X is
-   start of memory region reserved for dump-capture kernel.
+   second kernel using boot parameter "crashkernel=Y@X". Here X is start of memory region reserved for dump-capture kernel.
    Generally X is 16MB (0x1000000). So you can set
    CONFIG_PHYSICAL_START=0x1000000
 
@@ -315,6 +314,22 @@ 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 big 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, the CMA reservation should not be used if kdump is configured
+   to dump userspace memory - the re-used memory ranges won't be in the vmcore.
+
+   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
+   kdump is preferrable 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 fb8752b42ec8..ea4d53435515 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -987,6 +987,20 @@
 			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,
+			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.
+			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 kdump is preferrable 
+			to no kdump at all.
 
 	cryptomgr.notests
 			[KNL] Disable crypto self-tests

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