Per-process memory usage increased between kernel 4.4.x and 5.6

[Chris Packham]

Hi,

Not sure exactly where to direct this so I'm just hitting linux-mm and 
linux-kernel and hoping someone sees it.

We're in the process of updating our embedded targets from Linux 4.4.x 
to the latest kernel (currently v5.6 but we're planning to go to v5.7 
and maybe v5.8 depending on timing).

We've hit an issue where across multiple architectures (at least arm, 
mips and powerpc) we're seeing a significant decrease in the amount of 
Free/Available memory (40 to 90 MB). It seems to get worse with more 
processes running but no single process is consuming a significantly 
greater amount of memory.

Is anyone able to provide some hints as to where we should investigate 
or what in the kernel might have a per-process impact on memory used.

Thanks,
Chris