linux-kernel - [PATCH] x86/mm/tlb: ignore f->new_tlb

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Message-Id: <20220708003053.158480-1-namit@vmware.com>
Date:   Thu,  7 Jul 2022 17:30:53 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     linux-kernel@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Linux MM <linux-mm@...ck.org>,
        Nadav Amit <namit@...are.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH] x86/mm/tlb: ignore f->new_tlb_gen when zero

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

Commit aa44284960d5 ("x86/mm/tlb: Avoid reading mm_tlb_gen when
possible") introduced an optimization of skipping the flush if the TLB
generation that is flushed (as provided in flush_tlb_info) was already
flushed.

However, arch_tlbbatch_flush() does not provide any generation in
flush_tlb_info. As a result, try_to_unmap_one() would not perform any
TLB flushes.

Fix it by checking whether f->new_tlb_gen is nonzero. Zero value is
anyhow is an invalid generation value.

In addition, add the missing unlikely() and jump to get tracing right.

Fixes: aa44284960d5 ("x86/mm/tlb: Avoid reading mm_tlb_gen when possible")
Reported-by: Hugh Dickins <hughd@...gle.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Andy Lutomirski <luto@...nel.org>
Signed-off-by: Nadav Amit <namit@...are.com>
---
 arch/x86/mm/tlb.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index d9314cc8b81f..d81b4084bb8a 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -771,14 +771,14 @@ static void flush_tlb_func(void *info)
 		return;
 	}
 
-	if (f->new_tlb_gen <= local_tlb_gen) {
+	if (unlikely(f->new_tlb_gen != 0 && f->new_tlb_gen <= local_tlb_gen)) {
 		/*
 		 * The TLB is already up to date in respect to f->new_tlb_gen.
 		 * While the core might be still behind mm_tlb_gen, checking
 		 * mm_tlb_gen unnecessarily would have negative caching effects
 		 * so avoid it.
 		 */
-		return;
+		goto done;
 	}
 
 	/*
-- 
2.25.1