On Fri, Mar 07, 2025 at 02:30:47PM -0500, Mathieu Desnoyers wrote: > 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 with following changes: > > A) Use 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). > > B) Use test_and_set_bit_lock rather than test_and_set_bit to test-and-set > PGDAT_RECLAIM_LOCKED with an acquire memory ordering semantic. > > This changes the "lock" acquisition from a full barrier to an acquire > memory ordering, which is weaker. The acquire semi-permeable barrier > paired with the release on unlock is sufficient for this mutual > exclusion use-case. FWIW, this aligns with my understanding. Is (A) intended to be (submitted separately and) backported? Andrea > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> > Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> > Cc: Andrea Parri <parri.andrea@xxxxxxxxx> > Cc: Will Deacon <will@xxxxxxxxxx> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: Boqun Feng <boqun.feng@xxxxxxxxx> > Cc: Nicholas Piggin <npiggin@xxxxxxxxx> > Cc: David Howells <dhowells@xxxxxxxxxx> > Cc: Jade Alglave <j.alglave@xxxxxxxxx> > Cc: Luc Maranget <luc.maranget@xxxxxxxx> > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx> > Cc: linux-mm@xxxxxxxxx > --- > mm/vmscan.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index c22175120f5d..021b25bdba91 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -7567,11 +7567,11 @@ int node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned int order) > if (node_state(pgdat->node_id, N_CPU) && pgdat->node_id != numa_node_id()) > return NODE_RECLAIM_NOSCAN; > > - if (test_and_set_bit(PGDAT_RECLAIM_LOCKED, &pgdat->flags)) > + if (test_and_set_bit_lock(PGDAT_RECLAIM_LOCKED, &pgdat->flags)) > 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); > -- > 2.25.1 >
