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

Lomovtsev, Vadim Vadim.Lomovtsev at cavium.com
Wed Oct 24 05:52:49 PDT 2018

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.


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