[RFC PATCH v4 18/18] mm/swap: redistribute queue readers after membership changes

[Lian Wang <lianux.mm@gmail.com>]

Adding a device to an existing priority ring reuses the per-CPU reader
storage. A ring that previously contained one device can therefore keep
every CPU at the old common starting offset after peers are added.
Deleting devices or rings can likewise leave offsets and quotas that
describe the previous membership.

Reset the valid per-CPU ring offsets and quotas after peer insertion and
every nonempty deletion. Run the initialization helper under the topology
writer, which excludes allocation walks, without allocating new reader
storage or adding selector hot-path work.

Use cpu-id modulo placement for a fresh best-effort rotation across all
valid rings. This does not move existing slots or guarantee strict balance
for every active CPU subset. Mask and availability transitions do not
trigger the topology reset.

Signed-off-by: Lian Wang <lianux.mm@gmail.com>
---
 mm/swapfile.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 0c192844b121..eb2128132455 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -497,6 +497,7 @@ static int swap_queue_add(struct swap_info_struct *si,
 	spin_unlock(&swap_queue_update_lock);
 	memalloc_noreclaim_restore(noreclaim_flag);
 	kfree(old_ring);
+	swap_queue_reset_readers();
 	return 0;
 
 failed:
@@ -569,6 +570,8 @@ static void swap_queue_del(struct swap_info_struct *si)
 			swap_queue_install_readers(NULL, 0);
 	}
 	spin_unlock(&swap_queue_update_lock);
+	if (swap_queue_len)
+		swap_queue_reset_readers();
 
 	kfree(old_ring);
 	kfree(old_queue);