Re: [PATCH 02/20] mm: Add optional TLB flush to generic RCU page-table freeing

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Wed, 2012-06-27 at 15:23 -0700, Linus Torvalds wrote:

> Plus it really isn't about hardware page table walkers at all. It's
> more about the possibility of speculative TLB fils, it has nothing to
> do with *how* they are done. Sure, it's likely that a software
> pagetable walker wouldn't be something that gets called speculatively,
> but it's not out of the question.
> 
Hmm, I would call gup_fast() as speculative as we can get in software.
It does a lock-less walk of the page-tables. That's what the RCU free'd
page-table stuff is for to begin with.
> 
> IOW, if Sparc/PPC really want to guarantee that they never fill TLB
> entries speculatively, and that if we are in a kernel thread they will
> *never* fill the TLB with anything else, then make them enable
> CONFIG_STRICT_TLB_FILL or something in their architecture Kconfig
> files. 

Since we've dealt with the speculative software side by using RCU-ish
stuff, the only thing that's left is hardware, now neither sparc64 nor
ppc actually know about the linux page-tables from what I understood,
they only look at their hash-table thing.

So even if the hardware did do speculative tlb fills, it would do them
from the hash-table, but that's already cleared out.


How about something like this

---
Subject: mm: Add missing TLB invalidate to RCU page-table freeing
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Thu Jun 28 00:49:33 CEST 2012

For normal systems we need a TLB invalidate before freeing the
page-tables, the generic RCU based page-table freeing code lacked
this.

This is because this code originally came from ppc where the hardware
never walks the linux page-tables and thus this invalidate is not
required.

Others, notably s390 which ran into this problem in cd94154cc6a
("[S390] fix tlb flushing for page table pages"), do very much need
this TLB invalidation.

Therefore add it, with a Kconfig option to disable it so as to not
unduly slow down PPC and SPARC64 which neither of them need it.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 arch/Kconfig         |    3 +++
 arch/powerpc/Kconfig |    1 +
 arch/sparc/Kconfig   |    1 +
 mm/memory.c          |   18 ++++++++++++++++++
 4 files changed, 23 insertions(+)

--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -231,6 +231,9 @@ config HAVE_ARCH_MUTEX_CPU_RELAX
 config HAVE_RCU_TABLE_FREE
 	bool
 
+config STRICT_TLB_FILL
+	bool
+
 config ARCH_HAVE_NMI_SAFE_CMPXCHG
 	bool
 
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -127,6 +127,7 @@ config PPC
 	select GENERIC_IRQ_SHOW_LEVEL
 	select IRQ_FORCED_THREADING
 	select HAVE_RCU_TABLE_FREE if SMP
+	select STRICT_TLB_FILL
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_BPF_JIT if PPC64
 	select HAVE_ARCH_JUMP_LABEL
--- a/arch/sparc/Kconfig
+++ b/arch/sparc/Kconfig
@@ -52,6 +52,7 @@ config SPARC64
 	select HAVE_KRETPROBES
 	select HAVE_KPROBES
 	select HAVE_RCU_TABLE_FREE if SMP
+	select STRICT_TLB_FILL
 	select HAVE_MEMBLOCK
 	select HAVE_MEMBLOCK_NODE_MAP
 	select HAVE_SYSCALL_WRAPPERS
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -329,11 +329,27 @@ static void tlb_remove_table_rcu(struct 
 	free_page((unsigned long)batch);
 }
 
+#ifdef CONFIG_STRICT_TLB_FILL
+/*
+ * Some archictures (sparc64, ppc) cannot refill TLBs after the they've removed
+ * the PTE entries from their hash-table. Their hardware never looks at the
+ * linux page-table structures, so they don't need a hardware TLB invalidate
+ * when tearing down the page-table structure itself.
+ */
+static inline void tlb_table_flush_mmu(struct mmu_gather *tlb) { }
+#else
+static inline void tlb_table_flush_mmu(struct mmu_gather *tlb)
+{
+	tlb_flush_mmu(tlb);
+}
+#endif
+
 void tlb_table_flush(struct mmu_gather *tlb)
 {
 	struct mmu_table_batch **batch = &tlb->batch;
 
 	if (*batch) {
+		tlb_table_flush_mmu(tlb);
 		call_rcu_sched(&(*batch)->rcu, tlb_remove_table_rcu);
 		*batch = NULL;
 	}
@@ -345,6 +361,7 @@ void tlb_remove_table(struct mmu_gather 
 
 	tlb->need_flush = 1;
 
+#ifdef CONFIG_STRICT_TLB_FILL
 	/*
 	 * When there's less then two users of this mm there cannot be a
 	 * concurrent page-table walk.
@@ -353,6 +370,7 @@ void tlb_remove_table(struct mmu_gather 
 		__tlb_remove_table(table);
 		return;
 	}
+#endif
 
 	if (*batch == NULL) {
 		*batch = (struct mmu_table_batch *)__get_free_page(GFP_NOWAIT | __GFP_NOWARN);

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