Patch "mm: delete unnecessary and unsafe init_tlb_ubc()" has been added to the 4.7-stable tree

This is a note to let you know that I've just added the patch titled

mm: delete unnecessary and unsafe init_tlb_ubc()

to the 4.7-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-delete-unnecessary-and-unsafe-init_tlb_ubc.patch
and it can be found in the queue-4.7 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Hugh Dickins <[email protected]>
Date: Fri, 23 Sep 2016 20:27:04 -0700
Subject: mm: delete unnecessary and unsafe init_tlb_ubc()

From: Hugh Dickins <[email protected]>

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.

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

@@ -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]) {

Patches currently in stable-queue which might be from [email protected] are

queue-4.7/mm-fix-the-page_swap_info-bug_on-check.patch queue-4.7/ipc-shm-fix-crash-if-config_shmem-is-not-set.patch queue-4.7/mm-delete-unnecessary-and-unsafe-init_tlb_ubc.patch