linux-kernel - [PATCH 2/4] mm/tlb: Remove tlb_remove

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Message-ID: <20180822154046.772017055@infradead.org>
Date:   Wed, 22 Aug 2018 17:30:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     torvalds@...ux-foundation.org
Cc:     peterz@...radead.org, luto@...nel.org, x86@...nel.org,
        bp@...en8.de, will.deacon@....com, riel@...riel.com,
        jannh@...gle.com, ascannell@...gle.com, dave.hansen@...el.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Nicholas Piggin <npiggin@...il.com>,
        David Miller <davem@...emloft.net>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: [PATCH 2/4] mm/tlb: Remove tlb_remove_table() non-concurrent condition

Will noted that only checking mm_users is incorrect; we should also
check mm_count in order to cover CPUs that have a lazy reference to
this mm (and could do speculative TLB operations).

If removing this turns out to be a performance issue, we can
re-instate a more complete check, but in tlb_table_flush() eliding the
call_rcu_sched().

Cc: stable@...nel.org
Cc: Nicholas Piggin <npiggin@...il.com>
Cc: David Miller <davem@...emloft.net>
Cc: Will Deacon <will.deacon@....com>
Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
Cc: Michael Ellerman <mpe@...erman.id.au>
Fixes: 267239116987 ("mm, powerpc: move the RCU page-table freeing into generic code")
Reported-by: Will Deacon <will.deacon@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 mm/memory.c |    9 ---------
 1 file changed, 9 deletions(-)

--- a/mm/memory.c
+++ b/mm/memory.c
@@ -375,15 +375,6 @@ void tlb_remove_table(struct mmu_gather
 {
 	struct mmu_table_batch **batch = &tlb->batch;
 
-	/*
-	 * When there's less then two users of this mm there cannot be a
-	 * concurrent page-table walk.
-	 */
-	if (atomic_read(&tlb->mm->mm_users) < 2) {
-		__tlb_remove_table(table);
-		return;
-	}
-
 	if (*batch == NULL) {
 		*batch = (struct mmu_table_batch *)__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
 		if (*batch == NULL) {