Re: kdump failed because of hotplug memory adding in kdump kernel

[Yasuaki Ishimatsu]

(2014/01/10 16:11), Baoquan wrote:
> On 01/09/14 at 02:56pm, Toshi Kani wrote:
> > On Thu, 2014-01-09 at 16:27 -0500, Vivek Goyal wrote:
> > > On Thu, Jan 09, 2014 at 11:34:30AM -0700, Toshi Kani wrote:
> > > > On Thu, 2014-01-09 at 13:23 -0500, Vivek Goyal wrote:
> > > > > On Thu, Jan 09, 2014 at 10:24:25AM -0700, Toshi Kani wrote:
> > > > >
> > > > > [..]
> > > > > > > I think creating a new command line option is simpler as compared to
> > > > > > > creating a new flag in bootparam which in turn disables memory hotplug.
> > > > > > > More users can use that option. For example, if for some reason hotplug
> > > > > > > code is crashing, one can just disable it on command line as work around
> > > > > > > and move on.
> > > > > >
> > > > > > I do not have a strong opinion about having such option.  However, I
> > > > > > think it is more user friendly to keep the exactmap option works alone
> > > > > > on any platforms.
> > > > >
> > > > > I think we should create internally a variable which will disable memory
> > > > > hotplug. And set that variable based on memmap=exactmap, mem=X and also
> > > > > provide a way to disable memory hotplug directly using command line
> > > > > option.
> > > > >
> > > > > Current kexec-tools can use memmap=exactmap and be happy. I am writing
> > > > > a new kexec syscall and will not be using memmap=exactmap and would need
> > > > > to use that command line option to disable memory hotplug behavior.
> > > >
> > > > Sounds good to me.
> > >
> > > Nobody responded to my other question, so I would ask it again.
> > >
> > > Assume we have disabled hotplug memory in second kernel. First kernel
> > > saw hotplug memory and assume crash kernel reserved region came from
> > > there. We will pass this memory in bootparams to second kernel and it
> > > will show up in E820 map. It should still be accessible in second kernel,
> > > is that right?
> >
> > Yes.
> >
> > > Or there is some dependency on ACPI doing some magic before this memory
> > > range is available in second kernel?
> >
> > No.  The 1st kernel reserves the crash kernel region, which cannot be
> > hot-deleted.  So, this region continues to be accessible by the 2nd
> > kernel without any operation.

If my understanding is correct:

> Now what I understand is if a several memsection is reserved for
> crashkernel, then in 2nd kernel, they are just like normal memory.

correct.

> In ns
> object tree, they are not treated as hotplug memory.

wrong.
They are treated as hotplug memory. But the memory cannot hot removed
because the memory has kernel memory.

> Otherwise, any hotplug memory which is not reserved for 2nd kernel can
> be parsed and need be added as hotplug memory, and add them into movable
> zone.

wrong.
The memory is allocated as normal zone and it is offline.

> Am I right?

> The other question, e820 reserve is done earlier than acpi
> initialization, because acpi_early_init() invocation is very late in
> start_kernel(). Does that means at the very beginning all memorys are in
> e820, later when acpi_early_init is called, hotplug memory is detected,
> they will be moved to different place or need be marked with a specific
> flag?

No.

Thanks,
Yasuaki Ishimatsu

> > I am more curious to know how makedumpfile decides what memory ranges to
> > dump.  The 1st kernel may have performed memory hot-add / delete
> > operations before a crash, so it needs to know the valid physical
> > address range at the time of crash, and may not rely on the E820 map
> > from BIOS (which is stale).  Am I right to assume that makedumpfile gets
> > it from the page tables of the 1st kernel?
>
> makedumpfile just do the dump, what memory ranges to dump is decided in
> 1st kernel by kexec-tools. In 1st kernel, if kexec-tools executed, it
> will find all System Ram memorys which exclude the reserved regions for
> kdump kernel, then build a logical elf file, each load segment is one of
> these System Ram memory regions, its addr and length is written into the
> program header.
>
> Then makedumpfile just read this elf file, and read all of them and
> dump.
>
> If after kexec-tools execution and before crash, a hotplug memory is
> removed, udev will check this and trigger a kdump restart, kexec-tools
> is executed again, System Ram region information are stored. The logical
> file header will be passed to 2nd kernel.
>
>
> > Thanks,
> > -Toshi
> >
> >
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