在 2019年10月12日 20:16, Dave Young 写道:
> Hi Eric,
>
> On 10/12/19 at 06:26am, Eric W. Biederman wrote:
>> Lianbo Jiang <liji...@redhat.com> writes:
>>
>>> When the crashkernel kernel command line option is specified, the
>>> low 1MiB memory will always be reserved, which makes that the memory
>>> allocated later won't fall into the low 1MiB area, thereby, it's not
>>> necessary to create a backup region and also no need to copy the first
>>> 640k content to a backup region.
>>>
>>> Currently, the code related to the backup region can be safely removed,
>>> so lets clean up.
>>>
>>> Signed-off-by: Lianbo Jiang <liji...@redhat.com>
>>> ---
>>
>>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
>>> index eb651fbde92a..cc5774fc84c0 100644
>>> --- a/arch/x86/kernel/crash.c
>>> +++ b/arch/x86/kernel/crash.c
>>> @@ -173,8 +173,6 @@ void native_machine_crash_shutdown(struct pt_regs *regs)
>>>
>>> #ifdef CONFIG_KEXEC_FILE
>>>
>>> -static unsigned long crash_zero_bytes;
>>> -
>>> static int get_nr_ram_ranges_callback(struct resource *res, void *arg)
>>> {
>>> unsigned int *nr_ranges = arg;
>>> @@ -234,9 +232,15 @@ static int prepare_elf64_ram_headers_callback(struct
>>> resource *res, void *arg)
>>> {
>>> struct crash_mem *cmem = arg;
>>>
>>> - cmem->ranges[cmem->nr_ranges].start = res->start;
>>> - cmem->ranges[cmem->nr_ranges].end = res->end;
>>> - cmem->nr_ranges++;
>>> + if (res->start >= SZ_1M) {
>>> + cmem->ranges[cmem->nr_ranges].start = res->start;
>>> + cmem->ranges[cmem->nr_ranges].end = res->end;
>>> + cmem->nr_ranges++;
>>> + } else if (res->end > SZ_1M) {
>>> + cmem->ranges[cmem->nr_ranges].start = SZ_1M;
>>> + cmem->ranges[cmem->nr_ranges].end = res->end;
>>> + cmem->nr_ranges++;
>>> + }
>>
>> What is going on with this chunk? I can guess but this needs a clear
>> comment.
>
> Indeed it needs some code comment, this is based on some offline
> discussion. cat /proc/vmcore will give a warning because ioremap is
> mapping the system ram.
>
> We pass the first 1M to kdump kernel in e820 as system ram so that 2nd
> kernel can use the low 1M memory because for example the trampoline
> code.
>
Thank you, Eric and Dave. I will add the code comment as below if it would be
OK.
@@ -234,9 +232,20 @@ static int prepare_elf64_ram_headers_callback(struct
resource *res, void *arg)
{
struct crash_mem *cmem = arg;
- cmem->ranges[cmem->nr_ranges].start = res->start;
- cmem->ranges[cmem->nr_ranges].end = res->end;
- cmem->nr_ranges++;
+ /*
+ * Currently, pass the low 1MiB range to kdump kernel in e820
+ * as system ram so that kdump kernel can also use the low 1MiB
+ * memory due to the real mode trampoline code.
+ * And later, the low 1MiB range will be exclued from elf header,
+ * which will avoid remapping the 1MiB system ram when dumping
+ * vmcore.
+ */
+ if (res->start >= SZ_1M) {
+ cmem->ranges[cmem->nr_ranges].start = res->start;
+ cmem->ranges[cmem->nr_ranges].end = res->end;
+ cmem->nr_ranges++;
+ } else if (res->end > SZ_1M) {
+ cmem->ranges[cmem->nr_ranges].start = SZ_1M;
+ cmem->ranges[cmem->nr_ranges].end = res->end;
+ cmem->nr_ranges++;
+ }
return 0;
}
>>
>>>
>>> return 0;
>>> }
>>
>>> @@ -356,9 +337,12 @@ int crash_setup_memmap_entries(struct kimage *image,
>>> struct boot_params *params)
>>> memset(&cmd, 0, sizeof(struct crash_memmap_data));
>>> cmd.params = params;
>>>
>>> - /* Add first 640K segment */
>>> - ei.addr = image->arch.backup_src_start;
>>> - ei.size = image->arch.backup_src_sz;
>>> + /*
>>> + * Add the low memory range[0x1000, SZ_1M], skip
>>> + * the first zero page.
>>> + */
>>> + ei.addr = PAGE_SIZE;
>>> + ei.size = SZ_1M - PAGE_SIZE;
>>> ei.type = E820_TYPE_RAM;
>>> add_e820_entry(params, &ei);
>>
>> Likewise here. Why do we need a special case?
>> Why the magic with PAGE_SIZE?
>
> Good catch, the zero page part is useless, I think no other special
> reason, just assumed zero page is not usable, but it should be ok to
> remove the special handling, just pass 0 - 1M is good enough.
>> Thanks
> Dave
>
