mm: add missing release barrier on PGDAT_RECLAIM_LOCKED unlock

The PGDAT_RECLAIM_LOCKED bit is used to provide mutual exclusion of node
reclaim for struct pglist_data using a single bit.

It is "locked" with a test_and_set_bit (similarly to a try lock) which
provides full ordering with respect to loads and stores done within
__node_reclaim().

It is "unlocked" with clear_bit(), which does not provide any ordering
with respect to loads and stores done before clearing the bit.

The lack of clear_bit() memory ordering with respect to stores within
__node_reclaim() can cause a subsequent CPU to fail to observe stores from
a prior node reclaim.  This is not an issue in practice on TSO (e.g.
x86), but it is an issue on weakly-ordered architectures (e.g.  arm64).

Fix this by using clear_bit_unlock rather than clear_bit to clear
PGDAT_RECLAIM_LOCKED with a release memory ordering semantic.

This provides stronger memory ordering (release rather than relaxed).

Link: https://lkml.kernel.org/r/20250312141014.129725-1-mathieu.desnoyers@efficios.com
Fixes: d773ed6b856a ("mm: test and set zone reclaim lock before starting reclaim")
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Andrea Parri <parri.andrea@gmail.com>
Cc: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Jade Alglave <j.alglave@ucl.ac.uk>
Cc: Luc Maranget <luc.maranget@inria.fr>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index be00af3763b536483ab132060d028ab36389c58d..bbd3913e38879caab4c0d42dc51a57af8d4d8ebc 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -7581,7 +7581,7 @@ int node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned int order)
                return NODE_RECLAIM_NOSCAN;
 
        ret = __node_reclaim(pgdat, gfp_mask, order);
-       clear_bit(PGDAT_RECLAIM_LOCKED, &pgdat->flags);
+       clear_bit_unlock(PGDAT_RECLAIM_LOCKED, &pgdat->flags);
 
        if (ret)
                count_vm_event(PGSCAN_ZONE_RECLAIM_SUCCESS);