kerndat: Make pagemap check more robust against swapped out pages.

Fix test of whether the kernel exposes page frame numbers to cope with the
possibility that the top of the stack is swapped out, which was happening
in about one 1 out of 3 million runs.  This lead to a later failure when
trying to read the PFN of the zero page, after which criu would exit with
no error message.

Original-From: Ambrose Feinstein <ambrose@google.com>
Signed-off-by: Michał Mirosław <emmir@google.com>

criu/kerndat.c

@@ -55,10 +55,11 @@
 #include "util-caps.h"
 
 struct kerndat_s kdat = {};
+volatile int dummy_var;
 
 static int check_pagemap(void)
 {
-	int ret, fd;
+	int ret, fd, retry;
 	u64 pfn = 0;
 
 	fd = __open_proc(PROC_SELF, EPERM, O_RDONLY, "pagemap");
@@ -72,11 +73,24 @@ static int check_pagemap(void)
 		return -1;
 	}
 
-	/* Get the PFN of some present page. Stack is here, so try it :) */
-	ret = pread(fd, &pfn, sizeof(pfn), (((unsigned long)&ret) / page_size()) * sizeof(pfn));
-	if (ret != sizeof(pfn)) {
-		pr_perror("Can't read pagemap");
-		return -1;
+	retry = 3;
+	while (retry--) {
+		++dummy_var;
+		/* Get the PFN of a page likely to be present. */
+		ret = pread(fd, &pfn, sizeof(pfn), PAGE_PFN((uintptr_t)&dummy_var) * sizeof(pfn));
+		if (ret != sizeof(pfn)) {
+			pr_perror("Can't read pagemap");
+			return -1;
+		}
+		/* The page can be swapped out by the time the read occurs,
+		 * in which case the rest of the bits are a swap offset,
+		 * and can't be used to determine whether PFNs are visible.
+		 * Retry if this happens. */
+		if (pfn & PME_PRESENT)
+			break;
+		pr_warn("got non-present PFN %#lx for the dummy data page; %s\n", (unsigned long)pfn,
+			retry ? "retrying" : "giving up");
+		pfn = 0;
 	}
 
 	close(fd);