linux-kernel - [PATCH 4.7 33/69] mm: delete unnecessary and unsafe init_tlb

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Date:   Wed, 28 Sep 2016 11:05:15 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.7 33/69] mm: delete unnecessary and unsafe init_tlb_ubc()

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

------------------

From: Hugh Dickins <hughd@...gle.com>

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@...gle.com>
Acked-by: Mel Gorman <mgorman@...hsingularity.net>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

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

--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2150,23 +2150,6 @@ out:
 	}
 }

-#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
-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;
-}
-#else
-static inline void init_tlb_ubc(void)
-{
-}
-#endif /* CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH */
-
 /*
  * This is a basic per-zone page freer.  Used by both kswapd and direct reclaim.
  */
@@ -2202,8 +2185,6 @@ static void shrink_zone_memcg(struct zon
 	scan_adjusted = (global_reclaim(sc) && !current_is_kswapd() &&
 			 sc->priority == DEF_PRIORITY);

-	init_tlb_ubc();
-
 	blk_start_plug(&plug);
 	while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
 					nr[LRU_INACTIVE_FILE]) {