linux-kernel - [RFC 01/20] mm/tlb: fix fullmm semantics

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Message-Id: <20210131001132.3368247-2-namit@vmware.com>
Date:   Sat, 30 Jan 2021 16:11:13 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     Nadav Amit <namit@...are.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Nick Piggin <npiggin@...il.com>, x86@...nel.org
Subject: [RFC 01/20] mm/tlb: fix fullmm semantics

From: Nadav Amit <namit@...are.com>

fullmm in mmu_gather is supposed to indicate that the mm is torn-down
(e.g., on process exit) and can therefore allow certain optimizations.
However, tlb_finish_mmu() sets fullmm, when in fact it want to say that
the TLB should be fully flushed.

Change tlb_finish_mmu() to set need_flush_all and check this flag in
tlb_flush_mmu_tlbonly() when deciding whether a flush is needed.

Signed-off-by: Nadav Amit <namit@...are.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Will Deacon <will@...nel.org>
Cc: Yu Zhao <yuzhao@...gle.com>
Cc: Nick Piggin <npiggin@...il.com>
Cc: x86@...nel.org
---
 include/asm-generic/tlb.h | 2 +-
 mm/mmu_gather.c           | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index 2c68a545ffa7..eea113323468 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -420,7 +420,7 @@ static inline void tlb_flush_mmu_tlbonly(struct mmu_gather *tlb)
 	 * these bits.
 	 */
 	if (!(tlb->freed_tables || tlb->cleared_ptes || tlb->cleared_pmds ||
-	      tlb->cleared_puds || tlb->cleared_p4ds))
+	      tlb->cleared_puds || tlb->cleared_p4ds || tlb->need_flush_all))
 		return;
 
 	tlb_flush(tlb);
diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index 0dc7149b0c61..5a659d4e59eb 100644
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -323,7 +323,7 @@ void tlb_finish_mmu(struct mmu_gather *tlb)
 		 * On x86 non-fullmm doesn't yield significant difference
 		 * against fullmm.
 		 */
-		tlb->fullmm = 1;
+		tlb->need_flush_all = 1;
 		__tlb_reset_range(tlb);
 		tlb->freed_tables = 1;
 	}
-- 
2.25.1