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SubjectRe: [PATCH 4/5] mm/memory_hotplug: Simplify node_states_check_changes_online
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On 9/19/18 6:08 AM, Oscar Salvador wrote:
> From: Oscar Salvador <osalvador@suse.de>
>
> While looking at node_states_check_changes_online, I stumbled
> upon some confusing things.
>
> Right after entering the function, we find this:
>
> if (N_MEMORY == N_NORMAL_MEMORY)
> zone_last = ZONE_MOVABLE;
>
> This is wrong.
> N_MEMORY cannot really be equal to N_NORMAL_MEMORY.
> My guess is that this wanted to be something like:
>
> if (N_NORMAL_MEMORY == N_HIGH_MEMORY)
>
> to check if we have CONFIG_HIGHMEM.
>
> Later on, in the CONFIG_HIGHMEM block, we have:
>
> if (N_MEMORY == N_HIGH_MEMORY)
> zone_last = ZONE_MOVABLE;
>
> Again, this is wrong, and will never be evaluated to true.
>
> Besides removing these wrong if statements, I simplified
> the function a bit.
>
> Signed-off-by: Oscar Salvador <osalvador@suse.de>
> ---
> mm/memory_hotplug.c | 71 +++++++++++++++++------------------------------------
> 1 file changed, 23 insertions(+), 48 deletions(-)
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 131c08106d54..ab3c1de18c5d 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -687,61 +687,36 @@ static void node_states_check_changes_online(unsigned long nr_pages,
> struct zone *zone, struct memory_notify *arg)
> {
> int nid = zone_to_nid(zone);
> - enum zone_type zone_last = ZONE_NORMAL;
>
> /*
> - * If we have HIGHMEM or movable node, node_states[N_NORMAL_MEMORY]
> - * contains nodes which have zones of 0...ZONE_NORMAL,
> - * set zone_last to ZONE_NORMAL.
> - *
> - * If we don't have HIGHMEM nor movable node,
> - * node_states[N_NORMAL_MEMORY] contains nodes which have zones of
> - * 0...ZONE_MOVABLE, set zone_last to ZONE_MOVABLE.
> + * zone_for_pfn_range() can only return a zone within
> + * (0..ZONE_NORMAL] or ZONE_MOVABLE.

But what if that changes, will this function need to change as well?

> + * If the zone is within the range (0..ZONE_NORMAL],
> + * we need to check if:
> + * 1) We need to set the node for N_NORMAL_MEMORY
> + * 2) On CONFIG_HIGHMEM systems, we need to also set
> + * the node for N_HIGH_MEMORY.
> + * 3) On !CONFIG_HIGHMEM, we can disregard N_HIGH_MEMORY,
> + * as N_HIGH_MEMORY falls back to N_NORMAL_MEMORY.
> */
> - if (N_MEMORY == N_NORMAL_MEMORY)
> - zone_last = ZONE_MOVABLE;
>
> - /*
> - * if the memory to be online is in a zone of 0...zone_last, and
> - * the zones of 0...zone_last don't have memory before online, we will
> - * need to set the node to node_states[N_NORMAL_MEMORY] after
> - * the memory is online.
> - */
> - if (zone_idx(zone) <= zone_last && !node_state(nid, N_NORMAL_MEMORY))
> - arg->status_change_nid_normal = nid;
> - else
> - arg->status_change_nid_normal = -1;
> -
> -#ifdef CONFIG_HIGHMEM
> - /*
> - * If we have movable node, node_states[N_HIGH_MEMORY]
> - * contains nodes which have zones of 0...ZONE_HIGHMEM,
> - * set zone_last to ZONE_HIGHMEM.
> - *
> - * If we don't have movable node, node_states[N_NORMAL_MEMORY]
> - * contains nodes which have zones of 0...ZONE_MOVABLE,
> - * set zone_last to ZONE_MOVABLE.
> - */
> - zone_last = ZONE_HIGHMEM;
> - if (N_MEMORY == N_HIGH_MEMORY)
> - zone_last = ZONE_MOVABLE;
> + if (zone_idx(zone) <= ZONE_NORMAL) {
> + if (!node_state(nid, N_NORMAL_MEMORY))
> + arg->status_change_nid_normal = nid;
> + else
> + arg->status_change_nid_normal = -1;
>
> - if (zone_idx(zone) <= zone_last && !node_state(nid, N_HIGH_MEMORY))
> - arg->status_change_nid_high = nid;
> - else
> - arg->status_change_nid_high = -1;
> -#else
> - /*
> - * When !CONFIG_HIGHMEM, N_HIGH_MEMORY equals N_NORMAL_MEMORY
> - * so setting the node for N_NORMAL_MEMORY is enough.
> - */
> - arg->status_change_nid_high = -1;
> -#endif
> + if (IS_ENABLED(CONFIG_HIGHMEM)) {
> + if (!node_state(nid, N_HIGH_MEMORY))
> + arg->status_change_nid_high = nid;

Should not we have:
else
arg->status_change_nid_high = -1; ?

> + } else
> + arg->status_change_nid_high = -1;


I prefer to have braces in else part as well when if has braces.

> + }
>
> /*
> - * if the node don't have memory befor online, we will need to
> - * set the node to node_states[N_MEMORY] after the memory
> - * is online.
> + * if the node doesn't have memory before onlining it, we will need
> + * to set the node to node_states[N_MEMORY] after the memory
> + * gets onlined.
> */
> if (!node_state(nid, N_MEMORY))
> arg->status_change_nid = nid;
>

I think it is simpler to have something like this:

int nid = zone_to_nid(zone);

arg->status_change_nid_high = -1;
arg->status_change_nid = -1;
arg->status_change_nid = -1;

if (!node_state(nid, N_MEMORY))
arg->status_change_nid = nid;
if (zone_idx(zone) <= ZONE_NORMAL && !node_state(nid, N_NORMAL_MEMORY))
arg->status_change_nid_normal = nid;
#ifdef CONFIG_HIGHMEM
if (zone_idx(zone) <= N_HIGH_MEMORY && !node_state(nid, N_HIGH_MEMORY))
arg->status_change_nid_high = nid;
#endif


Pavel
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