X86/kdump: crashkernel=X try to reserve below 896M first then below 4G and MAXMEM

Now crashkernel=X will fail if there's not enough memory at low region (below 896M) when trying to reserve large memory size. One can use crashkernel=xM,high to reserve it at high region (>4G) but it is more convinient to improve crashkernel=X to:

- First try to reserve X below 896M (for being compatible with old kexec-tools).
- If fails, try to reserve X below 4G (swiotlb need to stay below 4G).
- If fails, try to reserve X from MAXMEM top down.

It's more transparent and user-friendly.

If crashkernel is large and the reserved is beyond 896M, old kexec-tools is not compatible with new kernel because old kexec-tools can not load kernel at high memory region, there was an old discussion below:
https://lkml.org/lkml/2013/10/15/601

But actually the behavior is consistent during my test. Suppose old kernel fail to reserve memory at low areas, kdump does not work because no meory reserved. With this patch, suppose new kernel successfully reserved memory at high areas, old kexec-tools still fail to load kdump kernel (tested 2.0.2), so it is acceptable, no need to worry about the compatibility.

Here is the test result (kexec-tools 2.0.2, no high memory load support):
Crashkernel over 4G:

# cat /proc/iomem|grep Crash
  be000000-cdffffff : Crash kernel
  213000000-21effffff : Crash kernel
# ./kexec  -p /boot/vmlinuz-`uname -r`
Memory for crashkernel is not reserved
Please reserve memory by passing "crashkernel=X@Y" parameter to the kernel
Then try loading kdump kernel

crashkernel: 896M-4G:

# cat /proc/iomem|grep Crash
  96000000-cdefffff : Crash kernel
# ./kexec -p /boot/vmlinuz-4.14.0-rc4+
ELF core (kcore) parse failed
Cannot load /boot/vmlinuz-4.14.0-rc4+

Signed-off-by Dave Young <[email protected]>

---
 arch/x86/kernel/setup.c |   16 ++++++++++++++++
 1 file changed, 16 insertions(+)

arch/x86/kernel/setup.c +9 -0

@@ -568,6 +568,22 @@ static void __init reserve_crashkernel(v
 						    high ? CRASH_ADDR_HIGH_MAX
 							 : CRASH_ADDR_LOW_MAX,
 						    crash_size, CRASH_ALIGN);
+#ifdef CONFIG_X86_64
+		/*
+		 * crashkernel=X reserve below 896M fails? Try below 4G
+		 */
+		if (!high && !crash_base)
+			crash_base = memblock_find_in_range(CRASH_ALIGN,
+						(1ULL << 32),
+						crash_size, CRASH_ALIGN);
+		/*

Now crashkernel=X will fail if there's not enough memory at low region (below 896M) when trying to reserve large memory size. One can use crashkernel=xM,high to reserve it at high region (>4G) but it is more convinient to improve crashkernel=X to:

- First try to reserve X below 896M (for being compatible with old kexec-tools).
- If fails, try to reserve X below 4G (swiotlb need to stay below 4G).
- If fails, try to reserve X from MAXMEM top down.

It's more transparent and user-friendly.

If crashkernel is large and the reserved is beyond 896M, old kexec-tools is not compatible with new kernel because old kexec-tools can not load kernel at high memory region, there was an old discussion below:
https://lkml.org/lkml/2013/10/15/601

But actually the behavior is consistent during my test. Suppose old kernel fail to reserve memory at low areas, kdump does not work because no meory reserved. With this patch, suppose new kernel successfully reserved memory at high areas, old kexec-tools still fail to load kdump kernel (tested 2.0.2), so it is acceptable, no need to worry about the compatibility.

Here is the test result (kexec-tools 2.0.2, no high memory load support):
Crashkernel over 4G:

# cat /proc/iomem|grep Crash
  be000000-cdffffff : Crash kernel
  213000000-21effffff : Crash kernel
# ./kexec  -p /boot/vmlinuz-`uname -r`
Memory for crashkernel is not reserved
Please reserve memory by passing "crashkernel=X@Y" parameter to the kernel
Then try loading kdump kernel

crashkernel: 896M-4G:

# cat /proc/iomem|grep Crash
  96000000-cdefffff : Crash kernel
# ./kexec -p /boot/vmlinuz-4.14.0-rc4+
ELF core (kcore) parse failed
Cannot load /boot/vmlinuz-4.14.0-rc4+

Signed-off-by Dave Young <[email protected]>

---
 arch/x86/kernel/setup.c |   16 ++++++++++++++++
 1 file changed, 16 insertions(+)

arch/x86/kernel/setup.c +5 -0

@@ -568,6 +568,22 @@ static void __init reserve_crashkernel(v
 						    high ? CRASH_ADDR_HIGH_MAX
 							 : CRASH_ADDR_LOW_MAX,
 						    crash_size, CRASH_ALIGN);
+#ifdef CONFIG_X86_64
+		/*
+		 * crashkernel=X reserve below 896M fails? Try below 4G
+		 */
+		if (!high && !crash_base)

arch/x86/kernel/setup.c +1 -0

+#endif
 		if (!crash_base) {
 			pr_info("crashkernel reservation failed - No suitable area found.\n");
 			return;

Hi Dave,

Thanks for your effort to push this into upstream. While I have one concern, please see the inline comments.

Quoted 58 lines · Dave Young

Now crashkernel=X will fail if there's not enough memory at low region (below 896M) when trying to reserve large memory size. One can use crashkernel=xM,high to reserve it at high region (>4G) but it is more convinient to improve crashkernel=X to:

- First try to reserve X below 896M (for being compatible with old kexec-tools).
- If fails, try to reserve X below 4G (swiotlb need to stay below 4G).
- If fails, try to reserve X from MAXMEM top down.

It's more transparent and user-friendly.

If crashkernel is large and the reserved is beyond 896M, old kexec-tools is not compatible with new kernel because old kexec-tools can not load kernel at high memory region, there was an old discussion below:
https://lkml.org/lkml/2013/10/15/601

But actually the behavior is consistent during my test. Suppose old kernel fail to reserve memory at low areas, kdump does not work because no meory reserved. With this patch, suppose new kernel successfully reserved memory at high areas, old kexec-tools still fail to load kdump kernel (tested 2.0.2), so it is acceptable, no need to worry about the compatibility.

Here is the test result (kexec-tools 2.0.2, no high memory load support):
Crashkernel over 4G:

# cat /proc/iomem|grep Crash
  be000000-cdffffff : Crash kernel
  213000000-21effffff : Crash kernel
# ./kexec  -p /boot/vmlinuz-`uname -r`
Memory for crashkernel is not reserved
Please reserve memory by passing "crashkernel=X@Y" parameter to the kernel
Then try loading kdump kernel

crashkernel: 896M-4G:

# cat /proc/iomem|grep Crash
  96000000-cdefffff : Crash kernel
# ./kexec -p /boot/vmlinuz-4.14.0-rc4+
ELF core (kcore) parse failed
Cannot load /boot/vmlinuz-4.14.0-rc4+

Signed-off-by Dave Young <[email protected]>

---
 arch/x86/kernel/setup.c |   16 ++++++++++++++++
 1 file changed, 16 insertions(+)

arch/x86/kernel/setup.c +5 -0

@@ -568,6 +568,22 @@ static void __init reserve_crashkernel(v
 						    high ? CRASH_ADDR_HIGH_MAX
 							 : CRASH_ADDR_LOW_MAX,
 						    crash_size, CRASH_ALIGN);
+#ifdef CONFIG_X86_64
+		/*
+		 * crashkernel=X reserve below 896M fails? Try below 4G
+		 */
+		if (!high && !crash_base)

arch/x86/kernel/setup.c static void __init reserve_crashkernel(v · line 585

Quoted 10 lines

arch/x86/kernel/setup.c +10 -0

+			crash_base = memblock_find_in_range(CRASH_ALIGN,
+						(1ULL << 32),
+						crash_size, CRASH_ALIGN);
+		/*
+		 * crashkernel=X reserve below 4G fails? Try MAXMEM
+		 */
+		if (!high && !crash_base)
+			crash_base = memblock_find_in_range(CRASH_ALIGN,
+						CRASH_ADDR_HIGH_MAX,
+						crash_size, CRASH_ALIGN);

For kdump, most of systems are x86 64. If both Yinghai and Vivek have no objection to search an available region of crash_size above 896M naturely, why don't we search it with function __memblock_find_range_bottom_up(). It can search from below 896M to above 4G, almost the same as the change you have made currently. Mainly the code will be much simpler.

The several times of searching looks not good and a little confusing.

What do you think?

Thanks
Baoquan

Quoted 6 lines

arch/x86/kernel/setup.c +1 -0

+#endif
 		if (!crash_base) {
 			pr_info("crashkernel reservation failed - No suitable area found.\n");
 			return;