[PATCH][2/2]page_fault retry with NOPAGE_RETRY

[Ying Han <yinghan@google.com>]

x86 support:

Signed-off-by: Ying Han <yinghan@google.com>
	       Mike Waychison <mikew@google.com>

arch/x86/mm/fault.c |   20 ++++++++++++++


diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 31e8730..0ec60a1 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -591,6 +591,7 @@ void __kprobes do_page_fault(struct pt_regs *regs, unsigne
 #ifdef CONFIG_X86_64
 	unsigned long flags;
 #endif
+	unsigned int retry_flag = FAULT_FLAG_RETRY;

 	tsk = current;
 	mm = tsk->mm;
@@ -689,6 +690,7 @@ again:
 		down_read(&mm->mmap_sem);
 	}

+retry:
 	vma = find_vma(mm, address);
 	if (!vma)
 		goto bad_area;
@@ -715,6 +717,7 @@ again:
 good_area:
 	si_code = SEGV_ACCERR;
 	write = 0;
+	write |= retry_flag;
 	switch (error_code & (PF_PROT|PF_WRITE)) {
 	default:	/* 3: write, present */
 		/* fall through */
@@ -743,6 +746,23 @@ good_area:
 			goto do_sigbus;
 		BUG();
 	}
+
+	/*
+	 * Here we retry fault once and switch to synchronous mode. The
+	 * main reason is to prevent us from the cases of starvation.
+	 * The retry logic open a starvation hole in which case pages might
+	 * be removed or changed after the retry.
+	 */
+	if (fault & VM_FAULT_RETRY) {
+		if (write & FAULT_FLAG_RETRY) {
+			retry_flag &= ~FAULT_FLAG_RETRY;
+			tsk->maj_flt++;
+			tsk->min_flt--;
+			goto retry;
+		}
+		BUG();
+	}
+
 	if (fault & VM_FAULT_MAJOR)
 		tsk->maj_flt++;
 	else

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