linux-kernel - Re: [patch 3/7] arch: mm: pass userspace fault flag to generic fault handler

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Date:	Mon, 5 Aug 2013 18:25:39 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Michal Hocko <mhocko@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	azurIt <azurit@...ox.sk>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm@...ck.org, cgroups@...r.kernel.org, x86@...nel.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 3/7] arch: mm: pass userspace fault flag to generic fault
 handler

On Mon, Aug 05, 2013 at 03:06:18PM -0700, Andrew Morton wrote:
> On Sat,  3 Aug 2013 12:59:56 -0400 Johannes Weiner <hannes@...xchg.org> wrote:
> 
> > Unlike global OOM handling, memory cgroup code will invoke the OOM
> > killer in any OOM situation because it has no way of telling faults
> > occuring in kernel context - which could be handled more gracefully -
> > from user-triggered faults.
> > 
> > Pass a flag that identifies faults originating in user space from the
> > architecture-specific fault handlers to generic code so that memcg OOM
> > handling can be improved.
> 
> arch/arm64/mm/fault.c has changed.  Here's what I came up with:
> 
> --- a/arch/arm64/mm/fault.c~arch-mm-pass-userspace-fault-flag-to-generic-fault-handler
> +++ a/arch/arm64/mm/fault.c
> @@ -199,13 +199,6 @@ static int __kprobes do_page_fault(unsig
>  	unsigned long vm_flags = VM_READ | VM_WRITE | VM_EXEC;
>  	unsigned int mm_flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
>  
> -	if (esr & ESR_LNX_EXEC) {
> -		vm_flags = VM_EXEC;
> -	} else if ((esr & ESR_WRITE) && !(esr & ESR_CM)) {
> -		vm_flags = VM_WRITE;
> -		mm_flags |= FAULT_FLAG_WRITE;
> -	}
> -
>  	tsk = current;
>  	mm  = tsk->mm;
>  
> @@ -220,6 +213,16 @@ static int __kprobes do_page_fault(unsig
>  	if (in_atomic() || !mm)
>  		goto no_context;
>  
> +	if (user_mode(regs))
> +		mm_flags |= FAULT_FLAG_USER;
> +
> +	if (esr & ESR_LNX_EXEC) {
> +		vm_flags = VM_EXEC;
> +	} else if ((esr & ESR_WRITE) && !(esr & ESR_CM)) {
> +		vm_flags = VM_WRITE;
> +		mm_flags |= FAULT_FLAG_WRITE;
> +	}
> +
>  	/*
>  	 * As per x86, we may deadlock here. However, since the kernel only
>  	 * validly references user space from well defined areas of the code,
> 
> But I'm not terribly confident in it.

It looks good to me.  They added the vm_flags but they are not used
any earlier than the mm_flags (__do_page_fault), which I moved to the
same location as you did in this fixup.

Thanks!
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