On 4/28/23 13:31, Hari Bathini wrote:
On 28/04/23 2:55 pm, Baoquan He wrote:
On 04/27/23 at 10:26pm, Hari Bathini wrote:
On 27/04/23 2:19 pm, Baoquan He wrote:
On 04/27/23 at 12:39pm, Hari Bathini wrote:
Hi Eric,
On 04/04/23 11:33 pm, Eric DeVolder wrote:
When CPU or memory is hot un/plugged, or off/onlined, the crash
elfcorehdr, which describes the CPUs and memory in the system,
must also be updated.
The segment containing the elfcorehdr is identified at run-time
in crash_core:crash_handle_hotplug_event(), which works for both
the kexec_load() and kexec_file_load() syscalls. A new elfcorehdr
is generated from the available CPUs and memory into a buffer,
and then installed over the top of the existing elfcorehdr.
In the patch 'kexec: exclude elfcorehdr from the segment digest'
the need to update purgatory due to the change in elfcorehdr was
eliminated. As a result, no changes to purgatory or boot_params
(as the elfcorehdr= kernel command line parameter pointer
remains unchanged and correct) are needed, just elfcorehdr.
To accommodate a growing number of resources via hotplug, the
elfcorehdr segment must be sufficiently large enough to accommodate
changes, see the CRASH_MAX_MEMORY_RANGES description. This is used
only on the kexec_file_load() syscall; for kexec_load() userspace
will need to size the segment similarly.
To accommodate kexec_load() syscall in the absence of
Firstly, thanks! This series is a nice improvement to kdump support
in hotplug environment.
Thank you!
One concern though is that this change assumes corresponding support
in kexec-tools. Without that support kexec_load would fail to boot
with digest verification failure, iiuc.
Yes, you've correctly identified that if a hotplug change occurs following kexec_load
(made with kexec-tools unaltered for hotplug), then a subsequent panic would in fact
fail the purgatory digest verification, and kdump would not happen.
Eric has posted patchset to modify kexec_tools to support that, please
see the link Eric pasted in the cover letter.
http://lists.infradead.org/pipermail/kexec/2022-October/026032.html
Right, Baoquan.
I did see that and if I read the code correctly, without that patchset
kexec_load would fail. Not with an explicit error that hotplug support
is missing or such but it would simply fail to boot into capture kernel
with digest verification failure.
This is correct.
My suggestion was to avoid that userspace tool breakage for older
kexec-tools version by introducing a new kexec flag that can tell
kernel that kexec-tools is ready to use this in-kernel update support.
So, if kexec_load happens without the flag, avoid doing an in-kernel
update on hotplug. I hope that clears the confusion.
Yeah, sounds like a good idea. It may be extended in later patch.
Fixing it in this series itself would be a cleaner way, I guess.
You're suggestion of using a flag makes alot of sense; it is an indication
to the kernel that it is valid/okay to modify the kexec_load elfcorehdr.
Only kexec-tools that understands this (meaning the elfcorehdr buffer is
appropriately sized *and* excludes the elfcorehdr from the purgatory check)
would set that flag.
The roll-out of this feature needs to be coordinated, no doubt. There are three
pieces to this puzzle: this kernel series, the udev rule changes, and the changes
to kexec-tools for kexec_load.
I consider the udev rule changes critical to making this feature work efficiently.
I also think that deploying the udev rules immediately is doable since nothing
references them, yet; they would be NOPs. And they would be in place when the
kernel and/or kexec-tool changes deploy.
However, your point about supporting kexec_load with and without this new flag
means the sysfs nodes upon which the udev rule change rely need to be a bit
smarter now. (I'm assuming these udev rules will be generally accepted as-is,
as they are simple and efficient.)
The sysfs crash_hotplug nodes need to take into account kexec_file_load vs
(kexec_load && new_flag). Generally speaking these crash_hotplug sysfs nodes we
want to be 1 going forward, but where kexec_load/kexec-tools is older and/or no new_flag,
it needs to be 0. In this way the udev rules can remain as proposed and work properly
for kexec_file_load and both flavors of kexec_load.
Good catch! I'll post v22 soon.
Eric
Thanks
Hari
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