nf_conntrack: falling back to vmalloc.

System log message file is reporting "kernel: nf_conntrack: falling back to vmalloc".

Memory page availability can be further checked from /proc/buddyinfo as below.

#  cat /proc/buddyinfo
Node 0, zone      DMA      1      0      1      0      1      1      1      0      1      1      3
Node 0, zone    DMA32   3342   2441   2138   5025   1871    236      3      0      0      0      0
Node 0, zone   Normal 143135   6057 150803   4005    330     62      1      0      0      0      0

Linux uses two methods to allocate memory

kmalloc
vmalloc

kmalloc allocates contiguous physical memory pages and it will fail if contiguous physical pages are not available.Then kernel falls back to vmalloc.

vmalloc is usually slower than kmalloc. It has to remap virtual memory range to physical space.

Once above message is reported from system log message. It highly indicates the memory is being fragmented.

Following command can be used to compact fragmented memory pages.

echo 1 > /proc/sys/vm/compact_memory

這個報錯不能算一個錯誤，只是說沒有足夠的連續內存可以分配kmalloc，因此啓用vmalloc來分配內存。

內存碎片化。