Since I’ve been using my 256MB Model B Pi as a server, I had been getting regular page allocation failures of the following kind:
sshd: page allocation failure: order:0, mode:0x20
This is rather annoying and seems to affect stability as well, like disrupted ssh or smb connections. It seems that the kernel has a setting to affect the minimum free memory that it keeps for allocations, which may be too low. So in the /etc/sysctl.conf file you can edit the last lines to bump this value from 8 Mbegabytes to 16 Megabytes:
# rpi tweaks
vm.min_free_kbytes = 16184
Settings are effective after a reboot. For me, this seems to fix the problem. But to be extra sure, I also changed the memory split from 192/64 to 224/32. Since my Pi runs headless, 64 MBytes seemed too much simply for a framebuffer console that is not even used. So after these tweaks my Pi shows 216 MByte of available memory and is running for some days now without page allocation errors.