Re: [PATCH 00/10] [v7][RESEND] Migrate Pages in lieu of discard

[Huang, Ying]

Michal Hocko <mhocko@xxxxxxxx> writes:

> On Fri 16-04-21 07:26:43, Dave Hansen wrote:
>> On 4/16/21 5:35 AM, Michal Hocko wrote:
>> > - Anonymous pages are bit tricky because they can be demoted even when
>> >   they cannot be reclaimed due to no (or no available) swap storage.
>> >   Unless I have missed something the second round will try to reclaim
>> >   them even the later is true and I am not sure this is completely OK.
>> 
>> What we want is something like this:
>> 
>> Swap Space / Demotion OK  -> Can Reclaim
>> Swap Space / Demotion Off -> Can Reclaim
>> Swap Full  / Demotion OK  -> Can Reclaim
>> Swap Full  / Demotion Off -> No Reclaim
>> 
>> I *think* that's what can_reclaim_anon_pages() ends up doing.  Maybe I'm
>> misunderstanding what you are referring to, though.  By "second round"
>> did you mean when we do reclaim on a node which is a terminal node?
>
> No, I mean the migration failure case where you splice back to the page
> list to reclaim. In that round you do not demote and want to reclaim.
> But a lack of swap space will make that page unreclaimable. I suspect
> this would just work out fine but I am not sure from the top of my head.

I have tested this via injecting some migration errors (returning 0 from
demote_page_list() before migration) on a system without swap.  The
system can still work properly.  In ftrace, I can find add_to_swap() are
called much more times, and it can deal with the situation where the
swap space isn't available.

Best Regards,
Huang, Ying