[PATCH 4.4 55/73] mm: delete unnecessary and unsafe init_tlb_ubc()

From: Greg Kroah-Hartman
Date: Wed Sep 28 2016 - 05:10:03 EST

4.4-stable review patch. If anyone has any objections, please let me know.


From: Hugh Dickins <hughd@xxxxxxxxxx>

commit b385d21f27d86426472f6ae92a231095f7de2a8d upstream.

init_tlb_ubc() looked unnecessary to me: tlb_ubc is statically
initialized with zeroes in the init_task, and copied from parent to
child while it is quiescent in arch_dup_task_struct(); so I went to
delete it.

But inserted temporary debug WARN_ONs in place of init_tlb_ubc() to
check that it was always empty at that point, and found them firing:
because memcg reclaim can recurse into global reclaim (when allocating
biosets for swapout in my case), and arrive back at the init_tlb_ubc()
in shrink_node_memcg().

Resetting tlb_ubc.flush_required at that point is wrong: if the upper
level needs a deferred TLB flush, but the lower level turns out not to,
we miss a TLB flush. But fortunately, that's the only part of the
protocol that does not nest: with the initialization removed, cpumask
collects bits from upper and lower levels, and flushes TLB when needed.

Fixes: 72b252aed506 ("mm: send one IPI per CPU to TLB flush all entries after unmapping pages")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

mm/vmscan.c | 19 -------------------
1 file changed, 19 deletions(-)

--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2159,23 +2159,6 @@ out:

-static void init_tlb_ubc(void)
- /*
- * This deliberately does not clear the cpumask as it's expensive
- * and unnecessary. If there happens to be data in there then the
- * first SWAP_CLUSTER_MAX pages will send an unnecessary IPI and
- * then will be cleared.
- */
- current->tlb_ubc.flush_required = false;
-static inline void init_tlb_ubc(void)
* This is a basic per-zone page freer. Used by both kswapd and direct reclaim.
@@ -2210,8 +2193,6 @@ static void shrink_lruvec(struct lruvec
scan_adjusted = (global_reclaim(sc) && !current_is_kswapd() &&
sc->priority == DEF_PRIORITY);

- init_tlb_ubc();