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    SubjectRe: [PATCH 0/6] mm: make movable onlining suck less
    On Thu 06-04-17 17:55:20, Mel Gorman wrote:
    > On Thu, Apr 06, 2017 at 06:21:55PM +0200, Michal Hocko wrote:
    > > > This was my first time using your git branch instead of applying the patches
    > > > from this thread to v4.11-rc5 myself.
    > >
    > > OK, so this looks like another thing to resolve. I have seen this
    > > warning as well but I didn't consider it relevant because I had to tweak
    > > the code make the node go offline (removed check_and_unmap_cpu_on_node
    > > from try_offline_node) so I thought it was a fallout from there.
    > >
    > > But let's have a look. hotadd_new_pgdat does for an existing pgdat
    > > /* Reset the nr_zones, order and classzone_idx before reuse */
    > > pgdat->nr_zones = 0;
    > > pgdat->kswapd_order = 0;
    > > pgdat->kswapd_classzone_idx = MAX_NR_ZONES;
    > >
    > > so free_area_init_node absolutely has to hit this warning. This is not
    > > in the Linus tree because it is still in Andrew's mmotm coming from
    > > http://www.ozlabs.org/~akpm/mmotm/broken-out/mm-vmscan-prevent-kswapd-sleeping-prematurely-due-to-mismatched-classzone_idx.patch
    > >
    > > So yay, finally that doesn't come from me. Mel, I guess that either
    > > hotadd_new_pgdat should keep its kswapd_classzone_idx = 0 or the warning
    > > should be updated.
    > >
    >
    > Actually, it's obvious very quickly when I started the fix that updating
    > the warning would then trigger on normal boot. It's more appropriate to
    > let a hotadd of a new pgdat defer the initialisation of that field to
    > kswapd starting for the new node.
    >
    > Can you try this? It's build/boot tested only, no hotplug testing.
    >
    > ---8<---
    > mm, vmscan: prevent kswapd sleeping prematurely due to mismatched classzone_idx -fix
    >
    > The patch "mm, vmscan: prevent kswapd sleeping prematurely due to mismatched
    > classzone_idx" has different initial starting conditions when kswapd
    > is asleep. kswapd initialises it properly when it starts but the patch
    > initialises kswapd_classzone_idx early and trips on a warning in
    > free_area_init_node. This patch leaves the kswapd_classzone_idx as zero
    > and defers to kswapd to initialise it properly when it starts.

    It will start during the online phase which is later than this physical
    memory hotadd.

    > This is a fix to the mmotm patch
    > mm-vmscan-prevent-kswapd-sleeping-prematurely-due-to-mismatched-classzone_idx.patch
    >
    > Signed-off-by: Mel Gorman <mgorman@suse.de>

    Yes, that is what I would expect. Feel free to add
    Acked-by: Michal Hocko <mhocko@suse.com>

    if this is routed as a separate patch. Although I expect Andrew will
    fold it into the original patch.

    > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
    > index 2309a7fbec93..76d4745513ee 100644
    > --- a/mm/memory_hotplug.c
    > +++ b/mm/memory_hotplug.c
    > @@ -1214,10 +1214,14 @@ static pg_data_t __ref *hotadd_new_pgdat(int nid, u64 start)
    >
    > arch_refresh_nodedata(nid, pgdat);
    > } else {
    > - /* Reset the nr_zones, order and classzone_idx before reuse */
    > + /*
    > + * Reset the nr_zones, order and classzone_idx before reuse.
    > + * Note that kswapd will init kswapd_classzone_idx properly
    > + * when it starts in the near future.
    > + */
    > pgdat->nr_zones = 0;
    > pgdat->kswapd_order = 0;
    > - pgdat->kswapd_classzone_idx = MAX_NR_ZONES;
    > + pgdat->kswapd_classzone_idx = 0;
    > }
    >
    > /* we can use NODE_DATA(nid) from here */
    >
    > --
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    --
    Michal Hocko
    SUSE Labs

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