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 15:06:18 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Johannes Weiner <hannes@...xchg.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 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.
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