linux-kernel - Re: [PATCH 2/2] s390/mm: allow gmap code to retry on faulting in guest memory

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Date:	Thu, 19 Nov 2015 09:18:08 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Dominik Dingel <dingel@...ux.vnet.ibm.com>
Cc:	linux-s390@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Eric B Munson <emunson@...mai.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Mel Gorman <mgorman@...e.de>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	"Jason J. Herne" <jjherne@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] s390/mm: allow gmap code to retry on faulting in
 guest memory

On Thu, 19 Nov 2015 00:49:58 +0100
Dominik Dingel <dingel@...ux.vnet.ibm.com> wrote:

> The userfaultfd does need FAULT_FLAG_ALLOW_RETRY to not return
> VM_FAULT_SIGBUS.  So we improve the gmap code to handle one
> VM_FAULT_RETRY.
> 
> Signed-off-by: Dominik Dingel <dingel@...ux.vnet.ibm.com>
> ---
>  arch/s390/mm/pgtable.c | 28 ++++++++++++++++++++++++----
>  1 file changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
> index 54ef3bc..8a0025d 100644
> --- a/arch/s390/mm/pgtable.c
> +++ b/arch/s390/mm/pgtable.c
> @@ -577,15 +577,22 @@ int gmap_fault(struct gmap *gmap, unsigned long gaddr,
>  	       unsigned int fault_flags)
>  {
>  	unsigned long vmaddr;
> -	int rc;
> +	int rc, fault;
> 
> +	fault_flags |= FAULT_FLAG_ALLOW_RETRY;
> +retry:
>  	down_read(&gmap->mm->mmap_sem);
>  	vmaddr = __gmap_translate(gmap, gaddr);
>  	if (IS_ERR_VALUE(vmaddr)) {
>  		rc = vmaddr;
>  		goto out_up;
>  	}
> -	if (fixup_user_fault(current, gmap->mm, vmaddr, fault_flags)) {
> +	fault = fixup_user_fault(current, gmap->mm, vmaddr, fault_flags);
> +	if (fault & VM_FAULT_RETRY) {
> +		fault_flags &= ~FAULT_FLAG_ALLOW_RETRY;
> +		fault_flags |= FAULT_FLAG_TRIED;
> +		goto retry;
> +	} else if (fault) {
>  		rc = -EFAULT;
>  		goto out_up;
>  	}

Me thinks that you want to add the retry code into fixup_user_fault itself.
You basically have the same code around the three calls to fixup_user_fault.
Yes, it will be a common code patch but I guess that it will be acceptable
given userfaultfd as a reason.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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