Chapter 2. Important Changes to External Kernel Parameters

This chapter provides system administrators with a summary of significant changes in the kernel shipped with Red Hat Enterprise Linux 6.7. These changes include added or updated procfs entries, sysfs default values, boot parameters, kernel configuration options, or any noticeable behavior changes.
efi_smbios_addr [X86,EFI]
Parameter used to specify location of SMBIOS for EFI systems. Used by kexec-tools for kdump.
initcall_blacklist [KNL]
A comma-separated list of initcall functions that should not be executed. Useful for debugging built-in modules and initcalls.
panic_on_warn
When enabled (set to 1), panic() is called after printing out the WARN() location. This is useful when you want to cause kdump on WARN().
/proc/<pid>/numa_maps
Shows memory location, binding policy, and mapping details of each mapping. Mapping details now include the page size in kilobytes (kernelpagesize_kB), in addition to mapping type, page usage counters, and node-based page counters.
/proc/<pid>/smaps
Shows memory consumption for each mapping of a process. The output now includes the kernel flags associated with the particular area of virtual memory (VmFlags). Kernel flags are shown as a space-separated list of two-letter codes:

Table 2.1. VmFlags Codes

CodeDescription
rdreadable
wrwriteable
exexecutable
shshared
mrmay read
mwmay write
memay execute
msmay share
gdstack segment growns down
pfpure PFN range
dwdisabled write to the mapped file
lopages are locked in memory
iomemory mapped I/O area
srsequential read advise provided
rrrandom read advise provided
dcdo not copy area on fork
dedo not expand area on remapping
acarea is accountable
nrswap space is not reserved for the area
htarea uses huge tlb pages
nlnon-linear mapping
ararchitecture specific flag
dddo not include area into core dump
mmmixed map area
hghuge page advise flag
nhno-huge page advise flag
mgmergable advise flag
net.ip.ip_no_pmtu_disc
Disables Path MTU Discovery. This parameter was previously Boolean; it now takes an Integer as a value. Possible values include:
0 (default)
Enables Path MTU Discovery.
1
Disables Path MTU Discovery by setting the PMTU to this destination to the value of min_pmtu when a fragmentation-requiring ICMP is received. To avoid locally-generated fragments, manually increase the value of min_pmtu to the interface on your system that has the smallest MTU.
2
Disables Path MTU Discovery by discarding Path MTU discovery messages. Outgoing frames are handled in the same way as in mode 1, implicitly setting IP_PMTUDISC_DONT on every created socket.
net.ip.ip_forward_use_pmtu
Disabled (set to 0) by default. When enabled, allows Path MTU Discovery while forwarding. Only enable this if you have user space software that depends on the kernel honoring Path MTU Discovery information.
net.core.netdev_rss_key
Contains a randomly generated host key used by drivers that enable RSS (Receive Side Scaling). Most drivers use 40 byte keys; this parameter allows keys up to 52 bytes. If no driver has ever called netdev_rss_key_fill(), this file contains null bytes.
vm.admin_reserve_kbytes
Defines the amount of free memory that should be reserved for users with the cap_sys_admin capability. On x86_64, the default value is 8 MB. This is sufficient when using the default overcommit mode. However, on systems where overcommit is set to never, this should be increased to account for the full size of recovery programs in virtual memory. On x86_64, the minimum useful reserve is about 128 MB. Changes to this parameter take effect whenever an application requests memory.