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Subject[PATCH v11 5/5] kdump: update Documentation about crashkernel
Date
Now the behavior of crashkernel=X has been changed, which tries low
allocation in ZONE_DMA, and fall back to high allocation if it fails.

If requized size X is too large and leads to very little free memory
in ZONE_DMA after low allocation, the system may not work well.
So add a threshold and go for high allocation directly if the required
size is too large. The threshold is set as the half of low memory.

If crash_base is outside ZONE_DMA, try to allocate at least 256M in
ZONE_DMA automatically. "crashkernel=Y,low" can be used to allocate
specified size low memory. For non-RPi4 platforms, change ZONE_DMA
memtioned above to ZONE_DMA32.

So update the Documentation.

Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
---
Documentation/admin-guide/kdump/kdump.rst | 21 ++++++++++++++++---
.../admin-guide/kernel-parameters.txt | 11 ++++++++--
2 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
index 2da65fef2a1c..4b58f97351d5 100644
--- a/Documentation/admin-guide/kdump/kdump.rst
+++ b/Documentation/admin-guide/kdump/kdump.rst
@@ -299,7 +299,15 @@ Boot into System Kernel
"crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
starting at physical address 0x01000000 (16MB) for the dump-capture kernel.

- On x86 and x86_64, use "crashkernel=64M@16M".
+ On x86 use "crashkernel=64M@16M".
+
+ On x86_64, use "crashkernel=X" to select a region under 4G first, and
+ fall back to reserve region above 4G.
+ We can also use "crashkernel=X,high" to select a region above 4G, which
+ also tries to allocate at least 256M below 4G automatically and
+ "crashkernel=Y,low" can be used to allocate specified size low memory.
+ Use "crashkernel=Y@X" if you really have to reserve memory from specified
+ start address X.

On ppc64, use "crashkernel=128M@32M".

@@ -316,8 +324,15 @@ Boot into System Kernel
kernel will automatically locate the crash kernel image within the
first 512MB of RAM if X is not given.

- On arm64, use "crashkernel=Y[@X]". Note that the start address of
- the kernel, X if explicitly specified, must be aligned to 2MiB (0x200000).
+ On arm64, use "crashkernel=X" to try low allocation in ZONE_DMA, and
+ fall back to high allocation if it fails. And go for high allocation
+ directly if the required size is too large. If crash_base is outside
+ ZONE_DMA, try to allocate at least 256M in ZONE_DMA automatically.
+ "crashkernel=Y,low" can be used to allocate specified size low memory.
+ For non-RPi4 platforms, change ZONE_DMA memtioned above to ZONE_DMA32.
+ Use "crashkernel=Y@X" if you really have to reserve memory from
+ specified start address X. Note that the start address of the kernel,
+ X if explicitly specified, must be aligned to 2MiB (0x200000).

Load the Dump-capture Kernel
============================
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index fb95fad81c79..d1b6016850d6 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -722,6 +722,10 @@
[KNL, x86_64] select a region under 4G first, and
fall back to reserve region above 4G when '@offset'
hasn't been specified.
+ [KNL, arm64] Try low allocation in ZONE_DMA, fall back
+ to high allocation if it fails when '@offset' hasn't been
+ specified. For non-RPi4 platforms, change ZONE_DMA to
+ ZONE_DMA32.
See Documentation/admin-guide/kdump/kdump.rst for further details.

crashkernel=range1:size1[,range2:size2,...][@offset]
@@ -746,13 +750,16 @@
requires at least 64M+32K low memory, also enough extra
low memory is needed to make sure DMA buffers for 32-bit
devices won't run out. Kernel would try to allocate at
- at least 256M below 4G automatically.
+ least 256M below 4G automatically.
This one let user to specify own low range under 4G
for second kernel instead.
0: to disable low allocation.
It will be ignored when crashkernel=X,high is not used
or memory reserved is below 4G.
-
+ [KNL, arm64] range under 4G.
+ This one let user to specify a low range in ZONE_DMA for
+ crash dump kernel. For non-RPi4 platforms, change ZONE_DMA
+ to ZONE_DMA32.
cryptomgr.notests
[KNL] Disable crypto self-tests

--
2.20.1
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