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@xxxxxxxxx>
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@xxxxxxxxx>
---
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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