[BUG] vmcore-dmesg cant' read dmesg log from /proc/vmcore if log_buf is reallocated due to large number of CPUs

Bhupesh Sharma bhsharma at redhat.com
Wed Oct 24 14:30:08 PDT 2018

Hello Vadim,

On Wed, Oct 24, 2018 at 6:23 PM Lomovtsev, Vadim
<Vadim.Lomovtsev at cavium.com> wrote:
> Hi all,
> Following issue has been found for vmcore-dmesg app with latest release (94159bc3c264fa26395e56302072276a139d18af 2.0.18-rc1) of kexec-tools at CentOS 7.5 distro:
> While having systems with large number of CPUs (e.g. Cavium ThunderX2 has 224) the log_buf gets reallocated by memblock_virt_alloc() at the setup_log_buf routine (https://elixir.bootlin.com/linux/v4.16.18/source/kernel/printk/printk.c#L1108).
> Then while dumping vmcore the vmcore-dmesg can't find dmesg log at /proc/vmcore file and exits with following message:
>   Failed to read log text of size 0 bytes: Bad address
> However it (vmcore-dmesg app) reads properly the log_buf symbol, it's address and eventually it's value from /proc/vmcore but fails to find dmesg data then.
> In the same time the makedumpfile is able to find and extract dmesg buffer from /proc/vmcore.
> The makedumpfile comes with kexec-tools-2.0.15-13.el7_5.2.aarch64 package.
> The issue is not reproduced for systems with small number of CPUs and log_buf not reallocated to memblock section.

Seems like you are hitting a known issue we saw on qualcomm amberwing
platforms as well.
I have sent a patch-series titled 'kexec-tools/arm64: Add support to
read PHYS_OFFSET from vmcoreinfo inside '/proc/kcore' to this list
just a few minutes back.

I have Cc'ed you to the patchset as I think it might fix the issue for
you. Kindly try the patchset on your platform (cavium?) and let me
know if this fixes the issue for you.


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