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    SubjectRe: [PATCH 00/10] [v7][RESEND] Migrate Pages in lieu of discard
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    Michal Hocko <mhocko@suse.com> 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

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