6.6-stable review patch. If anyone has any objections, please let me know.
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From: Mathieu Desnoyers mathieu.desnoyers@efficios.com
commit c0ebbb3841e07c4493e6fe351698806b09a87a37 upstream.
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... 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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/vmscan.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -8115,7 +8115,7 @@ int node_reclaim(struct pglist_data *pgd 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_FAILED);