[PATCH v2 1/2] mm/swap: Add VM_WARN_ON to isolate_lock_cluster()

[Hui Zhu]

From: Hui Zhu <zhuhui@xxxxxxxxxx>

swap_cluster_alloc_table() assumes that the caller holds the following
locks:
ci->lock
percpu_swap_cluster.lock
si->global_cluster_lock (required for non-SWP_SOLIDSTATE devices)

There are five call paths leading to swap_cluster_alloc_table():
swap_alloc_hibernation_slot->cluster_alloc_swap_entry
->alloc_swap_scan_list->isolate_lock_cluster->swap_cluster_alloc_table

swap_alloc_slow->cluster_alloc_swap_entry->alloc_swap_scan_list
->isolate_lock_cluster->swap_cluster_alloc_table

swap_alloc_hibernation_slot->cluster_alloc_swap_entry
->swap_reclaim_full_clusters->isolate_lock_cluster
->swap_cluster_alloc_table

swap_alloc_slow->cluster_alloc_swap_entry->swap_reclaim_full_clusters
->isolate_lock_cluster->swap_cluster_alloc_table

swap_reclaim_work->swap_reclaim_full_clusters->isolate_lock_cluster
->swap_cluster_alloc_table

Other paths correctly acquire the necessary locks before calling
swap_cluster_alloc_table().
But swap_reclaim_work() doesn't need do that because
swap_reclaim_full_clusters() just isolate si->full_clusters that
the tables of it must be allocated.
Then isolate_lock_cluster() that is called by
swap_reclaim_full_clusters() will never call swap_cluster_alloc_table().

This patch add a VM_WARN_ON to warning if a fill cluster will be handle
by swap_cluster_alloc_table()

Signed-off-by: Hui Zhu <zhuhui@xxxxxxxxxx>
---
 mm/swapfile.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 94af29d1de88..3fc2eb30c187 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -596,6 +596,9 @@ static struct swap_cluster_info *isolate_lock_cluster(
 	spin_unlock(&si->lock);
 
 	if (found && !cluster_table_is_alloced(found)) {
+		/* Table of full cluster must be allocated. */
+		VM_WARN_ON(ci->flags == CLUSTER_FLAG_FULL);
+
 		/* Only an empty free cluster's swap table can be freed. */
 		VM_WARN_ON_ONCE(list != &si->free_clusters);
 		VM_WARN_ON_ONCE(!cluster_is_empty(found));
-- 
2.43.0