What would be nice is to insert the information both during bootup and
in /proc/meminfo and 'free' output that hot-removable memory segments
are not generic free memory, it's currently a limited resource that
might or might not be sufficient to serve a given workload.
Perhaps even exclude it from 'total' memory reported by meminfo - to be
on the safe side of user expectations. In terms of user-space memory it
is already generic swappable memory but in terms of kernel-space
allocations it is not.
As i said it earlier in the thread, i certainly have no objections from
the x86 maintenance side - nothing is worse than a generic kernel
feature only available on certain less frequently used platforms. Memory
hotplug has been available for some time in the MM and it's not really
causing any maintenance trouble at the moment and it is not enabled by
default either.
Having said that, i have my doubts about its generic utility (the power
saving aspects are likely not realizable - nobody really wants DIMMs to
just sit there unused and the cost of dynamic migration is just
horrendous) - but as long as it's opt-in there's no reason to limit the
availability of an in-kernel feature artificially.
Removing those limitations of kernel-space allocations should indeed be
done in baby steps - and whether it's worth turning such memory into
completely generic kernel memory is an open question.
But the fact that a piece of memory is not fully generic is no reason
not to allow users to create special, capability-limited RAM resources
like they can already do via hugetlbfs or ramfs, as long as the the
capability limitations are advertised clearly.
Yes, memory hotplug has limitations we all understand, but still it's an
arguably useful feature in some circumstances. If we never give a
feature a chance to evolve on the main Linux platform that 90%+ of our
users use it wont ever be truly useful.
Please send the new patches against -git or -tip and we can put them
into a separate standalone feature topic and can test it on various x86
boxes and send them towards linux-next if Andrew agrees with that
process too.
Btw., it would be nice if memory hotplug had a self-test that could be
activated from the .config and would run autonomously (a bit like
rcu-torture): it would mark say 10% of all RAM as hot-pluggable during
bootup and would periodically hot-plug and hot-unplug that memory, every
10 seconds or 30 seconds or so, transparently. That would also test the
x86 architecture's pagetable init code, the page migration code, etc.
(Disabled by default and dependent on DEBUG_KERNEL && EXPERIMENTAL.)
Ingo
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