diff --git a/arch/um/os-Linux/mem.c b/arch/um/os-Linux/mem.c
index 3c4af77e51a2..897e9ad0c108 100644
--- a/arch/um/os-Linux/mem.c
+++ b/arch/um/os-Linux/mem.c
@@ -12,337 +12,117 @@
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/mman.h>
-#include <sys/param.h>
+#include <sys/vfs.h>
+#include <linux/magic.h>
 #include <init.h>
 #include <os.h>
 
-/* Modified by which_tmpdir, which is called during early boot */
-static char *default_tmpdir = "/tmp";
-
-/*
- *  Modified when creating the physical memory file and when checking
- * the tmp filesystem for usability, both happening during early boot.
- */
+/* Set by make_tempfile() during early boot. */
 static char *tempdir = NULL;
 
-static void __init find_tempdir(void)
+/* Check if dir is on tmpfs. Return 0 if yes, -1 if no or error. */
+static int __init check_tmpfs(const char *dir)
 {
-	const char *dirs[] = { "TMP", "TEMP", "TMPDIR", NULL };
-	int i;
-	char *dir = NULL;
+	struct statfs st;
 
-	if (tempdir != NULL)
-		/* We've already been called */
-		return;
-	for (i = 0; dirs[i]; i++) {
-		dir = getenv(dirs[i]);
-		if ((dir != NULL) && (*dir != '\0'))
-			break;
-	}
-	if ((dir == NULL) || (*dir == '\0'))
-		dir = default_tmpdir;
-
-	tempdir = malloc(strlen(dir) + 2);
-	if (tempdir == NULL) {
-		fprintf(stderr, "Failed to malloc tempdir, "
-			"errno = %d\n", errno);
-		return;
-	}
-	strcpy(tempdir, dir);
-	strcat(tempdir, "/");
-}
-
-/*
- * Remove bytes from the front of the buffer and refill it so that if there's a
- * partial string that we care about, it will be completed, and we can recognize
- * it.
- */
-static int pop(int fd, char *buf, size_t size, size_t npop)
-{
-	ssize_t n;
-	size_t len = strlen(&buf[npop]);
-
-	memmove(buf, &buf[npop], len + 1);
-	n = read(fd, &buf[len], size - len - 1);
-	if (n < 0)
-		return -errno;
-
-	buf[len + n] = '\0';
-	return 1;
-}
-
-/*
- * This will return 1, with the first character in buf being the
- * character following the next instance of c in the file.  This will
- * read the file as needed.  If there's an error, -errno is returned;
- * if the end of the file is reached, 0 is returned.
- */
-static int next(int fd, char *buf, size_t size, char c)
-{
-	ssize_t n;
-	char *ptr;
-
-	while ((ptr = strchr(buf, c)) == NULL) {
-		n = read(fd, buf, size - 1);
-		if (n == 0)
-			return 0;
-		else if (n < 0)
-			return -errno;
-
-		buf[n] = '\0';
-	}
-
-	return pop(fd, buf, size, ptr - buf + 1);
-}
-
-/*
- * Decode an octal-escaped and space-terminated path of the form used by
- * /proc/mounts. May be used to decode a path in-place. "out" must be at least
- * as large as the input. The output is always null-terminated. "len" gets the
- * length of the output, excluding the trailing null. Returns 0 if a full path
- * was successfully decoded, otherwise an error.
- */
-static int decode_path(const char *in, char *out, size_t *len)
-{
-	char *first = out;
-	int c;
-	int i;
-	int ret = -EINVAL;
-	while (1) {
-		switch (*in) {
-		case '\0':
-			goto out;
-
-		case ' ':
-			ret = 0;
-			goto out;
-
-		case '\\':
-			in++;
-			c = 0;
-			for (i = 0; i < 3; i++) {
-				if (*in < '0' || *in > '7')
-					goto out;
-				c = (c << 3) | (*in++ - '0');
-			}
-			*(unsigned char *)out++ = (unsigned char) c;
-			break;
-
-		default:
-			*out++ = *in++;
-			break;
-		}
-	}
-
-out:
-	*out = '\0';
-	*len = out - first;
-	return ret;
-}
-
-/*
- * Computes the length of s when encoded with three-digit octal escape sequences
- * for the characters in chars.
- */
-static size_t octal_encoded_length(const char *s, const char *chars)
-{
-	size_t len = strlen(s);
-	while ((s = strpbrk(s, chars)) != NULL) {
-		len += 3;
-		s++;
-	}
-
-	return len;
-}
-
-enum {
-	OUTCOME_NOTHING_MOUNTED,
-	OUTCOME_TMPFS_MOUNT,
-	OUTCOME_NON_TMPFS_MOUNT,
-};
-
-/* Read a line of /proc/mounts data looking for a tmpfs mount at "path". */
-static int read_mount(int fd, char *buf, size_t bufsize, const char *path,
-		      int *outcome)
-{
-	int found;
-	int match;
-	char *space;
-	size_t len;
-
-	enum {
-		MATCH_NONE,
-		MATCH_EXACT,
-		MATCH_PARENT,
-	};
-
-	found = next(fd, buf, bufsize, ' ');
-	if (found != 1)
-		return found;
-
-	/*
-	 * If there's no following space in the buffer, then this path is
-	 * truncated, so it can't be the one we're looking for.
-	 */
-	space = strchr(buf, ' ');
-	if (space) {
-		match = MATCH_NONE;
-		if (!decode_path(buf, buf, &len)) {
-			if (!strcmp(buf, path))
-				match = MATCH_EXACT;
-			else if (!strncmp(buf, path, len)
-				 && (path[len] == '/' || !strcmp(buf, "/")))
-				match = MATCH_PARENT;
-		}
-
-		found = pop(fd, buf, bufsize, space - buf + 1);
-		if (found != 1)
-			return found;
-
-		switch (match) {
-		case MATCH_EXACT:
-			if (!strncmp(buf, "tmpfs", strlen("tmpfs")))
-				*outcome = OUTCOME_TMPFS_MOUNT;
-			else
-				*outcome = OUTCOME_NON_TMPFS_MOUNT;
-			break;
-
-		case MATCH_PARENT:
-			/* This mount obscures any previous ones. */
-			*outcome = OUTCOME_NOTHING_MOUNTED;
-			break;
-		}
-	}
-
-	return next(fd, buf, bufsize, '\n');
-}
-
-/* which_tmpdir is called only during early boot */
-static int checked_tmpdir = 0;
-
-/*
- * Look for a tmpfs mounted at /dev/shm.  I couldn't find a cleaner
- * way to do this than to parse /proc/mounts.  statfs will return the
- * same filesystem magic number and fs id for both /dev and /dev/shm
- * when they are both tmpfs, so you can't tell if they are different
- * filesystems.  Also, there seems to be no other way of finding the
- * mount point of a filesystem from within it.
- *
- * If a /dev/shm tmpfs entry is found, then we switch to using it.
- * Otherwise, we stay with the default /tmp.
- */
-static void which_tmpdir(void)
-{
-	int fd;
-	int found;
-	int outcome;
-	char *path;
-	char *buf;
-	size_t bufsize;
-
-	if (checked_tmpdir)
-		return;
-
-	checked_tmpdir = 1;
-
-	printf("Checking for tmpfs mount on /dev/shm...");
-
-	path = realpath("/dev/shm", NULL);
-	if (!path) {
-		printf("failed to check real path, errno = %d\n", errno);
-		return;
-	}
-	printf("%s...", path);
-
-	/*
-	 * The buffer needs to be able to fit the full octal-escaped path, a
-	 * space, and a trailing null in order to successfully decode it.
-	 */
-	bufsize = octal_encoded_length(path, " \t\n\\") + 2;
-
-	if (bufsize < 128)
-		bufsize = 128;
-
-	buf = malloc(bufsize);
-	if (!buf) {
-		printf("malloc failed, errno = %d\n", errno);
-		goto out;
-	}
-	buf[0] = '\0';
-
-	fd = open("/proc/mounts", O_RDONLY);
-	if (fd < 0) {
-		printf("failed to open /proc/mounts, errno = %d\n", errno);
-		goto out1;
-	}
-
-	outcome = OUTCOME_NOTHING_MOUNTED;
-	while (1) {
-		found = read_mount(fd, buf, bufsize, path, &outcome);
-		if (found != 1)
-			break;
-	}
-
-	if (found < 0) {
-		printf("read returned errno %d\n", -found);
+	printf("Checking if %s is on tmpfs...", dir);
+	if (statfs(dir, &st) < 0) {
+		printf("%s\n", strerror(errno));
+	} else if (st.f_type != TMPFS_MAGIC) {
+		printf("no\n");
 	} else {
-		switch (outcome) {
-		case OUTCOME_TMPFS_MOUNT:
-			printf("OK\n");
-			default_tmpdir = "/dev/shm";
-			break;
-
-		case OUTCOME_NON_TMPFS_MOUNT:
-			printf("not tmpfs\n");
-			break;
-
-		default:
-			printf("nothing mounted on /dev/shm\n");
-			break;
-		}
+		printf("OK\n");
+		return 0;
 	}
-
-	close(fd);
-out1:
-	free(buf);
-out:
-	free(path);
+	return -1;
 }
 
-static int __init make_tempfile(const char *template, char **out_tempname,
-				int do_unlink)
+/*
+ * Choose the tempdir to use. We want something on tmpfs so that our memory is
+ * not subject to the host's vm.dirty_ratio. If a tempdir is specified in the
+ * environment, we use that even if it's not on tmpfs, but we warn the user.
+ * Otherwise, we try common tmpfs locations, and if no tmpfs directory is found
+ * then we fall back to /tmp.
+ */
+static char * __init choose_tempdir(void)
+{
+	static const char * const vars[] = {
+		"TMPDIR",
+		"TMP",
+		"TEMP",
+		NULL
+	};
+	static const char fallback_dir[] = "/tmp";
+	static const char * const tmpfs_dirs[] = {
+		"/dev/shm",
+		fallback_dir,
+		NULL
+	};
+	int i;
+	const char *dir;
+
+	printf("Checking environment variables for a tempdir...");
+	for (i = 0; vars[i]; i++) {
+		dir = getenv(vars[i]);
+		if ((dir != NULL) && (*dir != '\0')) {
+			printf("%s\n", dir);
+			if (check_tmpfs(dir) >= 0)
+				goto done;
+			else
+				goto warn;
+		}
+	}
+	printf("none found\n");
+
+	for (i = 0; tmpfs_dirs[i]; i++) {
+		dir = tmpfs_dirs[i];
+		if (check_tmpfs(dir) >= 0)
+			goto done;
+	}
+
+	dir = fallback_dir;
+warn:
+	printf("Warning: tempdir %s is not on tmpfs\n", dir);
+done:
+	/* Make a copy since getenv results may not remain valid forever. */
+	return strdup(dir);
+}
+
+/*
+ * Create an unlinked tempfile in a suitable tempdir. template must be the
+ * basename part of the template with a leading '/'.
+ */
+static int __init make_tempfile(const char *template)
 {
 	char *tempname;
 	int fd;
 
-	which_tmpdir();
-	tempname = malloc(MAXPATHLEN);
+	if (tempdir == NULL) {
+		tempdir = choose_tempdir();
+		if (tempdir == NULL) {
+			fprintf(stderr, "Failed to choose tempdir: %s\n",
+				strerror(errno));
+			return -1;
+		}
+	}
+
+	tempname = malloc(strlen(tempdir) + strlen(template) + 1);
 	if (tempname == NULL)
 		return -1;
 
-	find_tempdir();
-	if ((tempdir == NULL) || (strlen(tempdir) >= MAXPATHLEN))
-		goto out;
-
-	if (template[0] != '/')
-		strcpy(tempname, tempdir);
-	else
-		tempname[0] = '\0';
-	strncat(tempname, template, MAXPATHLEN-1-strlen(tempname));
+	strcpy(tempname, tempdir);
+	strcat(tempname, template);
 	fd = mkstemp(tempname);
 	if (fd < 0) {
 		fprintf(stderr, "open - cannot create %s: %s\n", tempname,
 			strerror(errno));
 		goto out;
 	}
-	if (do_unlink && (unlink(tempname) < 0)) {
+	if (unlink(tempname) < 0) {
 		perror("unlink");
 		goto close;
 	}
-	if (out_tempname) {
-		*out_tempname = tempname;
-	} else
-		free(tempname);
+	free(tempname);
 	return fd;
 close:
 	close(fd);
@@ -351,14 +131,14 @@ out:
 	return -1;
 }
 
-#define TEMPNAME_TEMPLATE "vm_file-XXXXXX"
+#define TEMPNAME_TEMPLATE "/vm_file-XXXXXX"
 
 static int __init create_tmp_file(unsigned long long len)
 {
 	int fd, err;
 	char zero;
 
-	fd = make_tempfile(TEMPNAME_TEMPLATE, NULL, 1);
+	fd = make_tempfile(TEMPNAME_TEMPLATE);
 	if (fd < 0)
 		exit(1);
 
@@ -402,7 +182,6 @@ int __init create_mem_file(unsigned long long len)
 	return fd;
 }
 
-
 void __init check_tmpexec(void)
 {
 	void *addr;
@@ -410,14 +189,13 @@ void __init check_tmpexec(void)
 
 	addr = mmap(NULL, UM_KERN_PAGE_SIZE,
 		    PROT_READ | PROT_WRITE | PROT_EXEC, MAP_PRIVATE, fd, 0);
-	printf("Checking PROT_EXEC mmap in %s...",tempdir);
-	fflush(stdout);
+	printf("Checking PROT_EXEC mmap in %s...", tempdir);
 	if (addr == MAP_FAILED) {
 		err = errno;
-		perror("failed");
+		printf("%s\n", strerror(err));
 		close(fd);
 		if (err == EPERM)
-			printf("%s must be not mounted noexec\n",tempdir);
+			printf("%s must be not mounted noexec\n", tempdir);
 		exit(1);
 	}
 	printf("OK\n");