Re: [PATCH 0/4] Swap migration V3: Overview

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

Andrew Morton wrote:
> Christoph Lameter <clameter@sgi.com> wrote:
> 
>>Page migration is also useful for other purposes:
>>
>> 1. Memory hotplug. Migrating processes off a memory node that is going
>>    to be disconnected.
>>
>> 2. Remapping of bad pages. These could be detected through soft ECC errors
>>    and other mechanisms.
> 
> 
> It's only useful for these things if it works with close-to-100% reliability.
> 
> And there are are all sorts of things which will prevent that - mlock,
> ongoing direct-io, hugepages, whatever.
> 
In lhms tree, current status is below: (If I'm wrong, plz fix)
==
For mlock, direct page migration will work fine. try_to_unmap_one()
in -mhp tree has an argument *force* and ignore VM_LOCKED, it's for this.

For direct-io, we have to wait for completion.
The end of I/O is not notified and memory_migrate() is just polling pages.

For hugepages, we'll need hugepage demand paging and more work, I think.
==

When a process migrates to other nodes by hand, it can cooperate with migration
subsystem. So we don't have to be afraid of some special using of memory, in many case.
I think Christoph's approach will work fine.

When it comes to memory-hotplug, arbitrary processes are affected.
It's more difficult.

We should focus on 'process migraion on demand', in this thread.

-- Kame