Re: [PATCH v2 2/4] mm: rmap: Allow platforms without mm_cpumask to defer TLB flush

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Hi Barry,

On 2022/7/11 11:46, Barry Song wrote:
From: Barry Song <v-songbaohua@xxxxxxxx>

Platforms like ARM64 have hareware TLB shootdown broadcast. They
don't maintain mm_cpumask but just send tlbi and related sync
instructions for TLB flush. task's mm_cpumask is normally empty
in this case. We also allow deferred TLB flush on this kind of
platforms.

Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>>
---
...
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
+

Here is another option to enable arch's tlbbatch defer

[1] https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20171101101735.2318-2-khandual@xxxxxxxxxxxxxxxxxx/

  	/* 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;



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