detect and fix page allocation failure

[cozzyd]

spidev sometimes fails to allocate kernel memory. Solution seems to be to drop caches (e.g. echo 3 > /sys/vm/drop_caches ). Detect spidev allocation failure (inability to open spi device) and try to fix automatically? (though need permissions...)

[cozzyd]

Problem likely mitigated by increasing sys.vm.min_free_kbytes to 8192. we'll see...

[cozzyd]

seems like not completely fixed... but not sure how to reliably detect that this is the problem rather than something else (check journalctl? that's a bit complex). Maybe rno-g-acq should do this instead if it can't open the radiant?

[cozzyd]

perhaps we need to play with fadvise? see e.g. https://stackoverflow.com/questions/3755765/what-posix-fadvise-args-for-sequential-file-write