Re: [RFC][PATCH 0/8] Migrate Pages in lieu of discard

[Dave Hansen]

On 6/30/20 11:36 AM, Shakeel Butt wrote:
>> This is part of a larger patch set.  If you want to apply these or
>> play with them, I'd suggest using the tree from here.  It includes
>> autonuma-based hot page promotion back to DRAM:
>>
>>         http://lkml.kernel.org/r/c3d6de4d-f7c3-b505-2e64-8ee5f70b2118@xxxxxxxxx
>>
>> This is also all based on an upstream mechanism that allows
>> persistent memory to be onlined and used as if it were volatile:
>>
>>         http://lkml.kernel.org/r/20190124231441.37A4A305@xxxxxxxxxxxxxxxxxx
>>
> I have a high level question. Given a reclaim request for a set of
> nodes, if there is no demotion path out of that set, should the kernel
> still consider the migrations within the set of nodes? 

OK, to be specific, we're talking about a case where we've arrived at
try_to_free_pages() and, say, all of the nodes on the system are set in
sc->nodemask?  Isn't the common case that all nodes are set in
sc->nodemask?  Since there is never a demotion path out of the set of
all nodes, the common case would be that there is no demotion path out
of a reclaim node set.

If that's true, I'd say that the kernel still needs to consider
migrations even within the set.