linux-kernel - [RFC PATCH v1 4/5] arm64/mm: Delay tlbi in contpte

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Message-ID: <20241211154611.40395-5-miko.lenczewski@arm.com>
Date: Wed, 11 Dec 2024 15:45:05 +0000
From: Mikołaj Lenczewski <miko.lenczewski@....com>
To: catalin.marinas@....com,
	will@...nel.org,
	corbet@....net,
	maz@...nel.org,
	oliver.upton@...ux.dev,
	joey.gouly@....com,
	suzuki.poulose@....com,
	yuzenghui@...wei.com
Cc: Mikołaj Lenczewski <miko.lenczewski@....com>,
	linux-arm-kernel@...ts.infradead.org,
	liunx-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	kvmarm@...r.kernel.org
Subject: [RFC PATCH v1 4/5] arm64/mm: Delay tlbi in contpte_convert() under BBML2

When converting a region via contpte_convert() to use mTHP, we have two
different goals. We have to mark each entry as contiguous, and we would
like to smear the dirty and young (access) bits across all entries in
the contiguous block. Currently, we do this by first accumulating the
dirty and young bits in the block, using an atomic
__ptep_get_and_clear() and the relevant pte_{dirty,young}() calls,
performing a tlbi, and finally smearing the correct bits across the
block using __set_ptes().

This approach works fine for BBM level 0, but with support for BBM level
2 we are allowed to reorder the tlbi to after setting the pagetable
entries. This reordering means that other threads will not see an
invalid pagetable entry, instead operating on stale data, until we have
performed our smearing and issued the invalidation. Avoiding this
invalid entry reduces faults in other threads, and thus improves
performance marginally (more so when there are more threads).

Signed-off-by: Mikołaj Lenczewski <miko.lenczewski@....com>
---
 arch/arm64/mm/contpte.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
index 55107d27d3f8..fc927be800ee 100644
--- a/arch/arm64/mm/contpte.c
+++ b/arch/arm64/mm/contpte.c
@@ -68,9 +68,13 @@ static void contpte_convert(struct mm_struct *mm, unsigned long addr,
 			pte = pte_mkyoung(pte);
 	}

-	__flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3);
+	if (!system_supports_bbml2())
+		__flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3);

 	__set_ptes(mm, start_addr, start_ptep, pte, CONT_PTES);
+
+	if (system_supports_bbml2())
+		__flush_tlb_range(&vma, start_addr, addr, PAGE_SIZE, true, 3);
 }

 void __contpte_try_fold(struct mm_struct *mm, unsigned long addr,
-- 
2.45.2