From: Nadav Amit <nadav.amit@gmail.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
Lukasz Anaczkowski <lukasz.anaczkowski@intel.com>,
LKML <linux-kernel@vger.kernel.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Andi Kleen <ak@linux.intel.com>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Michal Hocko <mhocko@suse.com>,
Andrew Morton <akpm@linux-foundation.org>,
"H. Peter Anvin" <hpa@zytor.com>,
harish.srinivasappa@intel.com, lukasz.odzioba@intel.com
Subject: Re: [PATCH] Linux VM workaround for Knights Landing A/D leak
Date: Tue, 14 Jun 2016 19:35:17 -0700 [thread overview]
Message-ID: <3878B27E-756E-4086-93BE-B4799A70570F@gmail.com> (raw)
In-Reply-To: <CALCETrWwFQ-NHfUw56QRDWFikD-v29OSwOqVdGLst6APGc4+bw@mail.gmail.com>
Andy Lutomirski <luto@amacapital.net> wrote:
> On Tue, Jun 14, 2016 at 2:37 PM, Dave Hansen
> <dave.hansen@linux.intel.com> wrote:
>> On 06/14/2016 01:16 PM, Nadav Amit wrote:
>>> Dave Hansen <dave.hansen@linux.intel.com> wrote:
>>>
>>>> On 06/14/2016 09:47 AM, Nadav Amit wrote:
>>>>> Lukasz Anaczkowski <lukasz.anaczkowski@intel.com> wrote:
>>>>>
>>>>>>> From: Andi Kleen <ak@linux.intel.com>
>>>>>>> +void fix_pte_leak(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
>>>>>>> +{
>>>>> Here there should be a call to smp_mb__after_atomic() to synchronize with
>>>>> switch_mm. I submitted a similar patch, which is still pending (hint).
>>>>>
>>>>>>> + if (cpumask_any_but(mm_cpumask(mm), smp_processor_id()) < nr_cpu_ids) {
>>>>>>> + trace_tlb_flush(TLB_LOCAL_SHOOTDOWN, TLB_FLUSH_ALL);
>>>>>>> + flush_tlb_others(mm_cpumask(mm), mm, addr,
>>>>>>> + addr + PAGE_SIZE);
>>>>>>> + mb();
>>>>>>> + set_pte(ptep, __pte(0));
>>>>>>> + }
>>>>>>> +}
>>>>
>>>> Shouldn't that barrier be incorporated in the TLB flush code itself and
>>>> not every single caller (like this code is)?
>>>>
>>>> It is insane to require individual TLB flushers to be concerned with the
>>>> barriers.
>>>
>>> IMHO it is best to use existing flushing interfaces instead of creating
>>> new ones.
>>
>> Yeah, or make these things a _little_ harder to get wrong. That little
>> snippet above isn't so crazy that we should be depending on open-coded
>> barriers to get it right.
>>
>> Should we just add a barrier to mm_cpumask() itself? That should stop
>> the race. Or maybe we need a new primitive like:
>>
>> /*
>> * Call this if a full barrier has been executed since the last
>> * pagetable modification operation.
>> */
>> static int __other_cpus_need_tlb_flush(struct mm_struct *mm)
>> {
>> /* cpumask_any_but() returns >= nr_cpu_ids if no cpus set. */
>> return cpumask_any_but(mm_cpumask(mm), smp_processor_id()) <
>> nr_cpu_ids;
>> }
>>
>>
>> static int other_cpus_need_tlb_flush(struct mm_struct *mm)
>> {
>> /*
>> * Synchronizes with switch_mm. Makes sure that we do not
>> * observe a bit having been cleared in mm_cpumask() before
>> * the other processor has seen our pagetable update. See
>> * switch_mm().
>> */
>> smp_mb__after_atomic();
>>
>> return __other_cpus_need_tlb_flush(mm)
>> }
>>
>> We should be able to deploy other_cpus_need_tlb_flush() in most of the
>> cases where we are doing "cpumask_any_but(mm_cpumask(mm),
>> smp_processor_id()) < nr_cpu_ids".
>
> IMO this is a bit nuts. smp_mb__after_atomic() doesn't do anything on
> x86. And, even if it did, why should the flush code assume that the
> previous store was atomic?
>
> What's the issue being fixed / worked around here?
It does a compiler barrier, which prevents the decision whether a
remote TLB shootdown is required to be made before the PTE is set.
I agree that PTEs may not be written atomically in certain cases
(although I am unaware of such cases, except on full-mm flush).
Having said that, I think that all the TLB flush/shootdown logic
should not be open-coded at all and be left in the arch-specific
implementation. People keep making small mistakes when they
reimplement the flushing logic.
This patch, for example, also has a bug in the way it traces the
flush - it marks full flush, when it flushes a single page:
>>>>>>> trace_tlb_flush(TLB_LOCAL_SHOOTDOWN, TLB_FLUSH_ALL);
Regards,
Nadav
next prev parent reply other threads:[~2016-06-15 2:35 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-14 15:58 [PATCH] Linux VM workaround for Knights Landing A/D leak Lukasz Anaczkowski
2016-06-14 16:31 ` kbuild test robot
2016-06-14 16:47 ` Nadav Amit
2016-06-14 16:54 ` Anaczkowski, Lukasz
2016-06-14 17:01 ` [PATCH v2] " Lukasz Anaczkowski
2016-06-14 17:24 ` Dave Hansen
2016-06-14 18:34 ` One Thousand Gnomes
2016-06-14 18:54 ` Dave Hansen
2016-06-14 19:19 ` Borislav Petkov
2016-06-14 20:20 ` H. Peter Anvin
2016-06-14 20:47 ` Borislav Petkov
2016-06-14 20:54 ` H. Peter Anvin
2016-06-14 21:02 ` Borislav Petkov
2016-06-14 21:08 ` H. Peter Anvin
2016-06-14 21:13 ` H. Peter Anvin
2016-06-14 18:10 ` Borislav Petkov
2016-06-15 13:12 ` Anaczkowski, Lukasz
2016-06-14 18:38 ` Nadav Amit
2016-06-15 13:12 ` Anaczkowski, Lukasz
2016-06-15 20:04 ` Nadav Amit
2016-06-15 20:10 ` Dave Hansen
2016-06-15 20:26 ` Nadav Amit
2016-06-16 15:14 ` [PATCH v3] " Lukasz Anaczkowski
2016-06-16 16:43 ` Nadav Amit
2016-06-16 20:23 ` Dave Hansen
2016-06-14 17:18 ` [PATCH] " Dave Hansen
2016-06-14 20:16 ` Nadav Amit
2016-06-14 21:37 ` Dave Hansen
2016-06-15 2:20 ` Andy Lutomirski
2016-06-15 2:35 ` Nadav Amit [this message]
2016-06-15 2:36 ` Andy Lutomirski
2016-06-15 2:44 ` Nadav Amit
2016-06-15 3:09 ` Andy Lutomirski
2016-06-15 3:20 ` Nadav Amit
2016-06-14 16:58 ` kbuild test robot
2016-06-14 17:19 ` Dave Hansen
2016-06-15 13:06 ` Anaczkowski, Lukasz
2016-06-14 17:47 ` kbuild test robot
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