The patch titled Subject: mm: add missing release barrier on PGDAT_RECLAIM_LOCKED unlock has been added to the -mm mm-unstable branch. Its filename is mm-add-missing-release-barrier-on-pgdat_reclaim_locked-unlock.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Mathieu Desnoyers mathieu.desnoyers@efficios.com Subject: mm: add missing release barrier on PGDAT_RECLAIM_LOCKED unlock Date: Wed, 12 Mar 2025 10:10:13 -0400

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 ---

mm/vmscan.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/vmscan.c~mm-add-missing-release-barrier-on-pgdat_reclaim_locked-unlock +++ a/mm/vmscan.c @@ -7581,7 +7581,7 @@ 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_SUCCESS); _

Patches currently in -mm which might be from mathieu.desnoyers@efficios.com are

mm-add-missing-release-barrier-on-pgdat_reclaim_locked-unlock.patch mm-lock-pgdat_reclaim_locked-with-acquire-memory-ordering.patch