Re: [PATCH] powerpc/crashkernel: take mem option into account

On Wed, Sep 18, 2019 at 7:23 PM Michael Ellerman [off-list ref] wrote:

Pingfan Liu [off-list ref] writes:

quoted

Cc Kexec list. And keep the original content.

On Thu, Sep 12, 2019 at 10:50 AM Pingfan Liu [off-list ref] wrote:

quoted

'mem=" option is an easy way to put high pressure on memory during some
test. Hence in stead of total mem, the effective usable memory size

               ^                          ^
               instead                    "actual" would be clearer

I think adding: "after applying the memory limit"

would help here.

quoted

quoted

should be considered when reserving mem for crashkernel. Otherwise
the boot up may experience oom issue.

                              ^
                              OOM

quoted

quoted

E.g passing
crashkernel="2G-4G:384M,4G-16G:512M,16G-64G:1G,64G-128G:2G,128G-:4G", and
mem=5G on a 256G machine.

Spelling out the behaviour before and after would help here, eg:

.. "would reserve 4G prior to the change and 512M afterward."

Thanks for kindly review. I will update the commit based on your suggestion.

quoted

quoted

Signed-off-by: Pingfan Liu <redacted>
Cc: Hari Bathini <hbathini@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
To: linuxppc-dev@lists.ozlabs.org
---
v1 -> v2: fix the printk info about the total mem
 arch/powerpc/kernel/machine_kexec.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/kernel/machine_kexec.c b/arch/powerpc/kernel/machine_kexec.c
index c4ed328..eec96dc 100644
--- a/arch/powerpc/kernel/machine_kexec.c
+++ b/arch/powerpc/kernel/machine_kexec.c

@@ -114,11 +114,12 @@ void machine_kexec(struct kimage *image)

 void __init reserve_crashkernel(void)
 {
-       unsigned long long crash_size, crash_base;
+       unsigned long long crash_size, crash_base, total_mem_sz;
        int ret;

+       total_mem_sz = memory_limit ? memory_limit : memblock_phys_mem_size();
        /* use common parsing */
-       ret = parse_crashkernel(boot_command_line, memblock_phys_mem_size(),
+       ret = parse_crashkernel(boot_command_line, total_mem_sz,
                        &crash_size, &crash_base);

I think this change makes sense. But we have multiple arches that
implement similar logic, and I wonder if we should keep them all the
same.

eg:

  arch/arm/kernel/setup.c:                ret = parse_crashkernel(boot_command_line, total_mem,
  arch/arm64/mm/init.c:                   ret = parse_crashkernel(boot_command_line, memblock_phys_mem_size(),
  arch/ia64/kernel/setup.c:               ret = parse_crashkernel(boot_command_line, total,
  arch/mips/kernel/setup.c:               ret = parse_crashkernel(boot_command_line, total_mem,
  arch/powerpc/kernel/fadump.c:           ret = parse_crashkernel(boot_command_line, memblock_phys_mem_size(),
  arch/powerpc/kernel/machine_kexec.c:    ret = parse_crashkernel(boot_command_line, memblock_phys_mem_size(),
  arch/s390/kernel/setup.c:               rc = parse_crashkernel(boot_command_line, memory_end, &crash_size,
  arch/sh/kernel/machine_kexec.c:         ret = parse_crashkernel(boot_command_line, memblock_phys_mem_size(),
  arch/x86/kernel/setup.c:                ret = parse_crashkernel(boot_command_line, total_mem, &crash_size, &crash_base);


From a quick glance most of them don't seem to take the memory limit
into account.

So I guess the question is do we want all arches to implement the same
behaviour or do we think it doesn't matter if they differ in details
like this?

On powerpc, the current code make fadump/kdump a higher priority than
"mem=" option, as the notes in fadump_reserve_mem() says
"
        /*
         * Calculate the memory boundary.
         * If memory_limit is less than actual memory boundary then reserve
         * the memory for fadump beyond the memory_limit and adjust the
         * memory_limit accordingly, so that the running kernel can run with
         * specified memory_limit.
         */
"

While on other archs, they pack "mem=" info into memblock before
calling memblock_phys_mem_size(). So when parse_crashkernel() calls
memblock_phys_mem_size(), the "mem=" takes effect.

E.g for x86 in arch/x86/kernel/e820.c
static int __init parse_memopt(char *p)
{
...
e820__range_remove(mem_size, ULLONG_MAX - mem_size, E820_TYPE_RAM, 1);
// this pack the "mem=" info into e820, and is finally feed to
memblock
}

Thanks,
Pingfan