linux-kernel - Re: [PATCH v2 19/20] x86/mm: Add speculative pagefault handling

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Message-Id: <dbaaf637-9f22-eba0-63a5-7bd83f810e4f@linux.vnet.ibm.com>
Date:   Mon, 21 Aug 2017 12:59:24 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>,
        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>
Cc:     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 v2 19/20] x86/mm: Add speculative pagefault handling

On 08/18/2017 03:35 AM, Laurent Dufour wrote:
> From: Peter Zijlstra <peterz@...radead.org>
> 
> Try a speculative fault before acquiring mmap_sem, if it returns with
> VM_FAULT_RETRY continue with the mmap_sem acquisition and do the
> traditional fault.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> 
> [Clearing of FAULT_FLAG_ALLOW_RETRY is now done in
>  handle_speculative_fault()]
> [Retry with usual fault path in the case VM_ERROR is returned by
>  handle_speculative_fault(). This allows signal to be delivered]
> Signed-off-by: Laurent Dufour <ldufour@...ux.vnet.ibm.com>
> ---
>  arch/x86/include/asm/pgtable_types.h |  7 +++++++
>  arch/x86/mm/fault.c                  | 19 +++++++++++++++++++
>  2 files changed, 26 insertions(+)
> 
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index bf9638e1ee42..4fd2693a037e 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -234,6 +234,13 @@ enum page_cache_mode {
>  #define PGD_IDENT_ATTR	 0x001		/* PRESENT (no other attributes) */
>  #endif
>  
> +/*
> + * Advertise that we call the Speculative Page Fault handler.
> + */
> +#ifdef CONFIG_X86_64
> +#define __HAVE_ARCH_CALL_SPF
> +#endif
> +
>  #ifdef CONFIG_X86_32
>  # include <asm/pgtable_32_types.h>
>  #else
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 2a1fa10c6a98..4c070b9a4362 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1365,6 +1365,24 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
>  	if (error_code & PF_INSTR)
>  		flags |= FAULT_FLAG_INSTRUCTION;
>  
> +#ifdef __HAVE_ARCH_CALL_SPF
> +	if (error_code & PF_USER) {
> +		fault = handle_speculative_fault(mm, address, flags);
> +
> +		/*
> +		 * We also check against VM_FAULT_ERROR because we have to
> +		 * raise a signal by calling later mm_fault_error() which
> +		 * requires the vma pointer to be set. So in that case,
> +		 * we fall through the normal path.

Cant mm_fault_error() be called inside handle_speculative_fault() ?
Falling through the normal page fault path again just to raise a
signal seems overkill. Looking into mm_fault_error(), it seems they
are different for x86 and powerpc.

X86:

mm_fault_error(struct pt_regs *regs, unsigned long error_code,
               unsigned long address, struct vm_area_struct *vma,
               unsigned int fault)

powerpc:

mm_fault_error(struct pt_regs *regs, unsigned long addr, int fault)

Even in case of X86, I guess we would have reference to the faulting
VMA (after the SRCU search) which can be used to call this function
directly.