linux-kernel - Re: [PATCH v9 23/24] x86/mm: Add speculative pagefault handling

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Date:   Mon, 26 Mar 2018 14:41:23 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>
cc:     paulmck@...ux.vnet.ibm.com, peterz@...radead.org,
        akpm@...ux-foundation.org, kirill@...temov.name,
        ak@...ux.intel.com, mhocko@...nel.org, dave@...olabs.net,
        jack@...e.cz, Matthew Wilcox <willy@...radead.org>,
        benh@...nel.crashing.org, mpe@...erman.id.au, paulus@...ba.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
        Will Deacon <will.deacon@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        kemi.wang@...el.com, sergey.senozhatsky.work@...il.com,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, khandual@...ux.vnet.ibm.com,
        npiggin@...il.com, bsingharora@...il.com,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [PATCH v9 23/24] x86/mm: Add speculative pagefault handling

On Tue, 13 Mar 2018, Laurent Dufour wrote:

> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index e6af2b464c3d..a73cf227edd6 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1239,6 +1239,9 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
>  		unsigned long address)
>  {
>  	struct vm_area_struct *vma;
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> +	struct vm_area_struct *spf_vma = NULL;
> +#endif
>  	struct task_struct *tsk;
>  	struct mm_struct *mm;
>  	int fault, major = 0;
> @@ -1332,6 +1335,27 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
>  	if (error_code & X86_PF_INSTR)
>  		flags |= FAULT_FLAG_INSTRUCTION;
>  
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> +	if ((error_code & X86_PF_USER) && (atomic_read(&mm->mm_users) > 1)) {
> +		fault = handle_speculative_fault(mm, address, flags,
> +						 &spf_vma);
> +
> +		if (!(fault & VM_FAULT_RETRY)) {
> +			if (!(fault & VM_FAULT_ERROR)) {
> +				perf_sw_event(PERF_COUNT_SW_SPF, 1,
> +					      regs, address);
> +				goto done;
> +			}
> +			/*
> +			 * In case of error we need the pkey value, but
> +			 * can't get it from the spf_vma as it is only returned
> +			 * when VM_FAULT_RETRY is returned. So we have to
> +			 * retry the page fault with the mmap_sem grabbed.
> +			 */
> +		}
> +	}
> +#endif /* CONFIG_SPECULATIVE_PAGE_FAULT */

All the comments from the powerpc version will apply here as well, the 
only interesting point is whether VM_FAULT_FALLBACK can be returned from 
handle_speculative_fault() to indicate its not possible.

> +
>  	/*
>  	 * When running in the kernel we expect faults to occur only to
>  	 * addresses in user space.  All other faults represent errors in
> @@ -1365,7 +1389,16 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
>  		might_sleep();
>  	}
>  
> -	vma = find_vma(mm, address);
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> +	if (spf_vma) {
> +		if (can_reuse_spf_vma(spf_vma, address))
> +			vma = spf_vma;
> +		else
> +			vma = find_vma(mm, address);
> +		spf_vma = NULL;
> +	} else
> +#endif
> +		vma = find_vma(mm, address);
>  	if (unlikely(!vma)) {
>  		bad_area(regs, error_code, address);
>  		return;
> @@ -1451,6 +1484,9 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
>  		return;
>  	}
>  
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> +done:
> +#endif
>  	/*
>  	 * Major/minor page fault accounting. If any of the events
>  	 * returned VM_FAULT_MAJOR, we account it as a major fault.