lkml.org 
[lkml]   [2017]   [Oct]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[tip:x86/urgent] x86/mm: Flush more aggressively in lazy TLB mode
Commit-ID:  b956575bed91ecfb136a8300742ecbbf451471ab
Gitweb: https://git.kernel.org/tip/b956575bed91ecfb136a8300742ecbbf451471ab
Author: Andy Lutomirski <luto@kernel.org>
AuthorDate: Mon, 9 Oct 2017 09:50:49 -0700
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Sat, 14 Oct 2017 09:21:24 +0200

x86/mm: Flush more aggressively in lazy TLB mode

Since commit:

94b1b03b519b ("x86/mm: Rework lazy TLB mode and TLB freshness tracking")

x86's lazy TLB mode has been all the way lazy: when running a kernel thread
(including the idle thread), the kernel keeps using the last user mm's
page tables without attempting to maintain user TLB coherence at all.

From a pure semantic perspective, this is fine -- kernel threads won't
attempt to access user pages, so having stale TLB entries doesn't matter.

Unfortunately, I forgot about a subtlety. By skipping TLB flushes,
we also allow any paging-structure caches that may exist on the CPU
to become incoherent. This means that we can have a
paging-structure cache entry that references a freed page table, and
the CPU is within its rights to do a speculative page walk starting
at the freed page table.

I can imagine this causing two different problems:

- A speculative page walk starting from a bogus page table could read
IO addresses. I haven't seen any reports of this causing problems.

- A speculative page walk that involves a bogus page table can install
garbage in the TLB. Such garbage would always be at a user VA, but
some AMD CPUs have logic that triggers a machine check when it notices
these bogus entries. I've seen a couple reports of this.

Boris further explains the failure mode:

> It is actually more of an optimization which assumes that paging-structure
> entries are in WB DRAM:
>
> "TlbCacheDis: cacheable memory disable. Read-write. 0=Enables
> performance optimization that assumes PML4, PDP, PDE, and PTE entries
> are in cacheable WB-DRAM; memory type checks may be bypassed, and
> addresses outside of WB-DRAM may result in undefined behavior or NB
> protocol errors. 1=Disables performance optimization and allows PML4,
> PDP, PDE and PTE entries to be in any memory type. Operating systems
> that maintain page tables in memory types other than WB- DRAM must set
> TlbCacheDis to insure proper operation."
>
> The MCE generated is an NB protocol error to signal that
>
> "Link: A specific coherent-only packet from a CPU was issued to an
> IO link. This may be caused by software which addresses page table
> structures in a memory type other than cacheable WB-DRAM without
> properly configuring MSRC001_0015[TlbCacheDis]. This may occur, for
> example, when page table structure addresses are above top of memory. In
> such cases, the NB will generate an MCE if it sees a mismatch between
> the memory operation generated by the core and the link type."
>
> I'm assuming coherent-only packets don't go out on IO links, thus the
> error.

To fix this, reinstate TLB coherence in lazy mode. With this patch
applied, we do it in one of two ways:

- If we have PCID, we simply switch back to init_mm's page tables
when we enter a kernel thread -- this seems to be quite cheap
except for the cost of serializing the CPU.

- If we don't have PCID, then we set a flag and switch to init_mm
the first time we would otherwise need to flush the TLB.

The /sys/kernel/debug/x86/tlb_use_lazy_mode debug switch can be changed
to override the default mode for benchmarking.

In theory, we could optimize this better by only flushing the TLB in
lazy CPUs when a page table is freed. Doing that would require
auditing the mm code to make sure that all page table freeing goes
through tlb_remove_page() as well as reworking some data structures
to implement the improved flush logic.

Reported-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Reported-by: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Eric Biggers <ebiggers@google.com>
Cc: Johannes Hirte <johannes.hirte@datenkhaos.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Roman Kagan <rkagan@virtuozzo.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 94b1b03b519b ("x86/mm: Rework lazy TLB mode and TLB freshness tracking")
Link: http://lkml.kernel.org/r/20171009170231.fkpraqokz6e4zeco@pd.tnic
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/include/asm/mmu_context.h | 8 +-
arch/x86/include/asm/tlbflush.h | 24 ++++++
arch/x86/mm/tlb.c | 153 +++++++++++++++++++++++++++----------
3 files changed, 136 insertions(+), 49 deletions(-)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index c120b5d..3c856a1 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -126,13 +126,7 @@ static inline void switch_ldt(struct mm_struct *prev, struct mm_struct *next)
DEBUG_LOCKS_WARN_ON(preemptible());
}

-static inline void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
-{
- int cpu = smp_processor_id();
-
- if (cpumask_test_cpu(cpu, mm_cpumask(mm)))
- cpumask_clear_cpu(cpu, mm_cpumask(mm));
-}
+void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk);

static inline int init_new_context(struct task_struct *tsk,
struct mm_struct *mm)
diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index 4893abf..d362161 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -83,6 +83,13 @@ static inline u64 inc_mm_tlb_gen(struct mm_struct *mm)
#endif

/*
+ * If tlb_use_lazy_mode is true, then we try to avoid switching CR3 to point
+ * to init_mm when we switch to a kernel thread (e.g. the idle thread). If
+ * it's false, then we immediately switch CR3 when entering a kernel thread.
+ */
+DECLARE_STATIC_KEY_TRUE(tlb_use_lazy_mode);
+
+/*
* 6 because 6 should be plenty and struct tlb_state will fit in
* two cache lines.
*/
@@ -105,6 +112,23 @@ struct tlb_state {
u16 next_asid;

/*
+ * We can be in one of several states:
+ *
+ * - Actively using an mm. Our CPU's bit will be set in
+ * mm_cpumask(loaded_mm) and is_lazy == false;
+ *
+ * - Not using a real mm. loaded_mm == &init_mm. Our CPU's bit
+ * will not be set in mm_cpumask(&init_mm) and is_lazy == false.
+ *
+ * - Lazily using a real mm. loaded_mm != &init_mm, our bit
+ * is set in mm_cpumask(loaded_mm), but is_lazy == true.
+ * We're heuristically guessing that the CR3 load we
+ * skipped more than makes up for the overhead added by
+ * lazy mode.
+ */
+ bool is_lazy;
+
+ /*
* Access to this CR4 shadow and to H/W CR4 is protected by
* disabling interrupts when modifying either one.
*/
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 49d9778..658bf00 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -30,6 +30,8 @@

atomic64_t last_mm_ctx_id = ATOMIC64_INIT(1);

+DEFINE_STATIC_KEY_TRUE(tlb_use_lazy_mode);
+
static void choose_new_asid(struct mm_struct *next, u64 next_tlb_gen,
u16 *new_asid, bool *need_flush)
{
@@ -80,7 +82,7 @@ void leave_mm(int cpu)
return;

/* Warn if we're not lazy. */
- WARN_ON(cpumask_test_cpu(smp_processor_id(), mm_cpumask(loaded_mm)));
+ WARN_ON(!this_cpu_read(cpu_tlbstate.is_lazy));

switch_mm(NULL, &init_mm, NULL);
}
@@ -142,45 +144,24 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
__flush_tlb_all();
}
#endif
+ this_cpu_write(cpu_tlbstate.is_lazy, false);

if (real_prev == next) {
VM_BUG_ON(this_cpu_read(cpu_tlbstate.ctxs[prev_asid].ctx_id) !=
next->context.ctx_id);

- if (cpumask_test_cpu(cpu, mm_cpumask(next))) {
- /*
- * There's nothing to do: we weren't lazy, and we
- * aren't changing our mm. We don't need to flush
- * anything, nor do we need to update CR3, CR4, or
- * LDTR.
- */
- return;
- }
-
- /* Resume remote flushes and then read tlb_gen. */
- cpumask_set_cpu(cpu, mm_cpumask(next));
- next_tlb_gen = atomic64_read(&next->context.tlb_gen);
-
- if (this_cpu_read(cpu_tlbstate.ctxs[prev_asid].tlb_gen) <
- next_tlb_gen) {
- /*
- * Ideally, we'd have a flush_tlb() variant that
- * takes the known CR3 value as input. This would
- * be faster on Xen PV and on hypothetical CPUs
- * on which INVPCID is fast.
- */
- this_cpu_write(cpu_tlbstate.ctxs[prev_asid].tlb_gen,
- next_tlb_gen);
- write_cr3(build_cr3(next, prev_asid));
- trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH,
- TLB_FLUSH_ALL);
- }
-
/*
- * We just exited lazy mode, which means that CR4 and/or LDTR
- * may be stale. (Changes to the required CR4 and LDTR states
- * are not reflected in tlb_gen.)
+ * We don't currently support having a real mm loaded without
+ * our cpu set in mm_cpumask(). We have all the bookkeeping
+ * in place to figure out whether we would need to flush
+ * if our cpu were cleared in mm_cpumask(), but we don't
+ * currently use it.
*/
+ if (WARN_ON_ONCE(real_prev != &init_mm &&
+ !cpumask_test_cpu(cpu, mm_cpumask(next))))
+ cpumask_set_cpu(cpu, mm_cpumask(next));
+
+ return;
} else {
u16 new_asid;
bool need_flush;
@@ -199,10 +180,9 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
}

/* Stop remote flushes for the previous mm */
- if (cpumask_test_cpu(cpu, mm_cpumask(real_prev)))
- cpumask_clear_cpu(cpu, mm_cpumask(real_prev));
-
- VM_WARN_ON_ONCE(cpumask_test_cpu(cpu, mm_cpumask(next)));
+ VM_WARN_ON_ONCE(!cpumask_test_cpu(cpu, mm_cpumask(real_prev)) &&
+ real_prev != &init_mm);
+ cpumask_clear_cpu(cpu, mm_cpumask(real_prev));

/*
* Start remote flushes and then read tlb_gen.
@@ -233,6 +213,37 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
}

/*
+ * enter_lazy_tlb() is a hint from the scheduler that we are entering a
+ * kernel thread or other context without an mm. Acceptable implementations
+ * include doing nothing whatsoever, switching to init_mm, or various clever
+ * lazy tricks to try to minimize TLB flushes.
+ *
+ * The scheduler reserves the right to call enter_lazy_tlb() several times
+ * in a row. It will notify us that we're going back to a real mm by
+ * calling switch_mm_irqs_off().
+ */
+void enter_lazy_tlb(struct mm_struct *mm, struct task_struct *tsk)
+{
+ if (this_cpu_read(cpu_tlbstate.loaded_mm) == &init_mm)
+ return;
+
+ if (static_branch_unlikely(&tlb_use_lazy_mode)) {
+ /*
+ * There's a significant optimization that may be possible
+ * here. We have accurate enough TLB flush tracking that we
+ * don't need to maintain coherence of TLB per se when we're
+ * lazy. We do, however, need to maintain coherence of
+ * paging-structure caches. We could, in principle, leave our
+ * old mm loaded and only switch to init_mm when
+ * tlb_remove_page() happens.
+ */
+ this_cpu_write(cpu_tlbstate.is_lazy, true);
+ } else {
+ switch_mm(NULL, &init_mm, NULL);
+ }
+}
+
+/*
* Call this when reinitializing a CPU. It fixes the following potential
* problems:
*
@@ -303,16 +314,20 @@ static void flush_tlb_func_common(const struct flush_tlb_info *f,
/* This code cannot presently handle being reentered. */
VM_WARN_ON(!irqs_disabled());

+ if (unlikely(loaded_mm == &init_mm))
+ return;
+
VM_WARN_ON(this_cpu_read(cpu_tlbstate.ctxs[loaded_mm_asid].ctx_id) !=
loaded_mm->context.ctx_id);

- if (!cpumask_test_cpu(smp_processor_id(), mm_cpumask(loaded_mm))) {
+ if (this_cpu_read(cpu_tlbstate.is_lazy)) {
/*
- * We're in lazy mode -- don't flush. We can get here on
- * remote flushes due to races and on local flushes if a
- * kernel thread coincidentally flushes the mm it's lazily
- * still using.
+ * We're in lazy mode. We need to at least flush our
+ * paging-structure cache to avoid speculatively reading
+ * garbage into our TLB. Since switching to init_mm is barely
+ * slower than a minimal flush, just switch to init_mm.
*/
+ switch_mm_irqs_off(NULL, &init_mm, NULL);
return;
}

@@ -611,3 +626,57 @@ static int __init create_tlb_single_page_flush_ceiling(void)
return 0;
}
late_initcall(create_tlb_single_page_flush_ceiling);
+
+static ssize_t tlblazy_read_file(struct file *file, char __user *user_buf,
+ size_t count, loff_t *ppos)
+{
+ char buf[2];
+
+ buf[0] = static_branch_likely(&tlb_use_lazy_mode) ? '1' : '0';
+ buf[1] = '\n';
+
+ return simple_read_from_buffer(user_buf, count, ppos, buf, 2);
+}
+
+static ssize_t tlblazy_write_file(struct file *file,
+ const char __user *user_buf, size_t count, loff_t *ppos)
+{
+ bool val;
+
+ if (kstrtobool_from_user(user_buf, count, &val))
+ return -EINVAL;
+
+ if (val)
+ static_branch_enable(&tlb_use_lazy_mode);
+ else
+ static_branch_disable(&tlb_use_lazy_mode);
+
+ return count;
+}
+
+static const struct file_operations fops_tlblazy = {
+ .read = tlblazy_read_file,
+ .write = tlblazy_write_file,
+ .llseek = default_llseek,
+};
+
+static int __init init_tlb_use_lazy_mode(void)
+{
+ if (boot_cpu_has(X86_FEATURE_PCID)) {
+ /*
+ * Heuristic: with PCID on, switching to and from
+ * init_mm is reasonably fast, but remote flush IPIs
+ * as expensive as ever, so turn off lazy TLB mode.
+ *
+ * We can't do this in setup_pcid() because static keys
+ * haven't been initialized yet, and it would blow up
+ * badly.
+ */
+ static_branch_disable(&tlb_use_lazy_mode);
+ }
+
+ debugfs_create_file("tlb_use_lazy_mode", S_IRUSR | S_IWUSR,
+ arch_debugfs_dir, NULL, &fops_tlblazy);
+ return 0;
+}
+late_initcall(init_tlb_use_lazy_mode);
\
 
 \ /
  Last update: 2017-10-14 13:02    [W:0.132 / U:63.860 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site