linux-kernel - [RFC PATCH v12 based on mm-unstable as of Feb 21, 2025 04/25] x86/tlb, riscv/tlb, mm/rmap: separate arch_tlbbatch_clear() out of arch_tlbbatch

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Message-Id: <20250226120132.28469-4-byungchul@sk.com>
Date: Wed, 26 Feb 2025 21:01:11 +0900
From: Byungchul Park <byungchul@...com>
To: linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Cc: kernel_team@...ynix.com,
	akpm@...ux-foundation.org,
	vernhao@...cent.com,
	mgorman@...hsingularity.net,
	hughd@...gle.com,
	willy@...radead.org,
	david@...hat.com,
	peterz@...radead.org,
	luto@...nel.org,
	tglx@...utronix.de,
	mingo@...hat.com,
	bp@...en8.de,
	rjgolo@...il.com
Subject: [RFC PATCH v12 based on mm-unstable as of Feb 21, 2025 04/25]  x86/tlb, riscv/tlb, mm/rmap: separate arch_tlbbatch_clear() out of arch_tlbbatch_flush()

A new mechanism, LUF(Lazy Unmap Flush), defers tlb flush until folios
that have been unmapped and freed, eventually get allocated again.  It's
safe for folios that had been mapped read only and were unmapped, since
the contents of the folios don't change while staying in pcp or buddy
so we can still read the data through the stale tlb entries.

This is a preparation for the mechanism that requires to avoid redundant
tlb flush by manipulating tlb batch's arch data.  To achieve that, we
need to separate the part clearing the tlb batch's arch data out of
arch_tlbbatch_flush().

Signed-off-by: Byungchul Park <byungchul@...com>
---
 arch/riscv/mm/tlbflush.c | 1 -
 arch/x86/mm/tlb.c        | 2 --
 mm/rmap.c                | 1 +
 3 files changed, 1 insertion(+), 3 deletions(-)

diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
index 74dd9307fbf1b..38f4bea8a964a 100644
--- a/arch/riscv/mm/tlbflush.c
+++ b/arch/riscv/mm/tlbflush.c
@@ -200,5 +200,4 @@ void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
 {
 	__flush_tlb_range(&batch->cpumask, FLUSH_TLB_NO_ASID, 0,
 			  FLUSH_TLB_MAX_SIZE, PAGE_SIZE);
-	cpumask_clear(&batch->cpumask);
 }
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 6cf881a942bbe..523e8bb6fba1f 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -1292,8 +1292,6 @@ void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
 		local_irq_enable();
 	}
 
-	cpumask_clear(&batch->cpumask);
-
 	put_flush_tlb_info();
 	put_cpu();
 }
diff --git a/mm/rmap.c b/mm/rmap.c
index bcec8677f68df..546b7a6a30a44 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -648,6 +648,7 @@ void try_to_unmap_flush(void)
 		return;
 
 	arch_tlbbatch_flush(&tlb_ubc->arch);
+	arch_tlbbatch_clear(&tlb_ubc->arch);
 	tlb_ubc->flush_required = false;
 	tlb_ubc->writable = false;
 }
-- 
2.17.1