linux, memory (mis)accounting/reporting, and the planner/optimizer

Apologies if this is a FAQ, but...

Given linux's (mis)accounting/reporting of per-process memory, including
shared memory (see for example this page:
http://lwn.net/Articles/230975/) how does postgresql interpret and use
the information that's provided?  Does it use the information as-is?
Does it just accept the configuration parameters provided (e.g. --
shared_buffers, effective_cache_size, etc.)?  Or does it combine its
internal knowledge of what it's sharing and adjust what linux reports
accordingly?

I'm aware that there are lots of userspace tools for accessing what the
kernel reports, but I think its reporting is still problematic,
especially for apps that use shared memory.  (User space tools like sar,
ps, top, pmap, free, vmstat, iostat, slabinfo, et al., as well as just
looking at /proc fds -- /proc/meminfo, etc.)