linux-kernel - Re: [PATCH v2] x86: enable RCU based table free

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Date:   Thu, 24 Aug 2017 12:49:36 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jork Loeser <Jork.Loeser@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v2] x86: enable RCU based table free

On Thu, Aug 24, 2017 at 11:22:58AM +0200, Vitaly Kuznetsov wrote:
> On x86 software page-table walkers depend on the fact that remote TLB flush
> does an IPI: walk is performed lockless but with interrupts disabled and in
> case the pagetable is freed the freeing CPU will get blocked as remote TLB
> flush is required. On other architecture which don't require an IPI to do
> remote TLB flush we have an RCU-based mechanism (see
> include/asm-generic/tlb.h for more details).
> 
> In virtualized environments we may want to override .flush_tlb_others hook
> in pv_mmu_ops and use a hypercall asking the hypervisor to do remote TLB
> flush for us. This breaks the assumption about IPI. Xen PV does this for
> years and the upcoming remote TLB flush for Hyper-V will do it too. This
> is not safe, software pagetable walkers may step on an already freed page.
> 
> Solve the issue by enabling RCU-based table free mechanism. Testing with
> kernbench and mmap/munmap microbenchmars didn't show any notable
> performance impact.
> 
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> Acked-by: Juergen Gross <jgross@...e.com>
> ---
> Changes since v1:
> - Enable HAVE_RCU_TABLE_FREE unconditionally to avoid different code pathes
>   for no reason [Linus Torvalds]

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
 Kirill A. Shutemov