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From:   Barry Song <21cnbao@gmail.com>
To:     namit@vmware.com
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        catalin.marinas@arm.com, corbet@lwn.net,
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Subject: Re: [PATCH 2/4] mm: rmap: Allow platforms without mm_cpumask to defer TLB flush
Date:   Fri,  8 Jul 2022 18:59:57 +1200
Message-Id: <20220708065957.3859-1-21cnbao@gmail.com>
In-Reply-To: <67943544-6DAA-4C8A-A04D-029D2D36C318@vmware.com>
References: <67943544-6DAA-4C8A-A04D-029D2D36C318@vmware.com>
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> The cpumask_empty() is indeed just another memory access, which is most
> likely ok. But wouldn’t adding something like CONFIG_ARCH_HAS_MM_CPUMASK
> make the code simpler and (slightly, certainly slightly) more performant?

Yep. good suggestion, Nadav. So the code will be as below, right?

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index be0b95e51df6..a91d73866238 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -81,6 +81,7 @@ config X86
 	select ARCH_HAS_KCOV			if X86_64
 	select ARCH_HAS_MEM_ENCRYPT
 	select ARCH_HAS_MEMBARRIER_SYNC_CORE
+	select ARCH_HAS_MM_CPUMASK
 	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
 	select ARCH_HAS_PMEM_API		if X86_64
 	select ARCH_HAS_PTE_DEVMAP		if X86_64
diff --git a/mm/Kconfig b/mm/Kconfig
index 169e64192e48..7bf54f57ca01 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -951,6 +951,9 @@ config ARCH_HAS_CURRENT_STACK_POINTER
 	  register alias named "current_stack_pointer", this config can be
 	  selected.

+config ARCH_HAS_MM_CPUMASK
+	bool
+
 config ARCH_HAS_VM_GET_PAGE_PROT
 	bool

diff --git a/mm/rmap.c b/mm/rmap.c
index 5bcb334cd6f2..13d4f9a1d4f1 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -692,6 +692,10 @@ static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags)
 	if (!(flags & TTU_BATCH_FLUSH))
 		return false;

+#ifndef CONFIG_ARCH_HAS_MM_CPUMASK
+	return true;
+#endif
+
 	/* If remote CPUs need to be flushed then defer batch the flush */
 	if (cpumask_any_but(mm_cpumask(mm), get_cpu()) < nr_cpu_ids)
 		should_defer = true;

Thanks
Barry