On 08/10/20 at 11:28am, chenzhou wrote:

On 2020/8/8 18:02, Dave Young wrote:

quoted

On 08/01/20 at 09:08pm, Chen Zhou wrote:

quoted

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 <redacted>
---
 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

Hi Chen,

Previously I remember we talked about to use similar logic as X86, but I
remember you mentioned on some arm64 platform there could be no low
memory at all.  Is this not a problem now for the fallback?  Just be
curious, thanks for the update, for the common part looks good.

Hi Dave,

Did you mean this discuss: https://lkml.org/lkml/2019/12/27/122?
This is about the different implementation instead of no low memory in arm64.

On arm64 platform, if there is no low memory, system will boot fail.

James mentioned some systems have no memory below 4G, if I understand it
correctly that means they can boot without low mem.

Anyway I like the new implementation in this series if it is good enough
for arm64 people.

Thanks,
Chen Zhou

quoted

Acked-by: Dave Young <redacted>

Thanks
Dave


.

Thanks
Dave


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