diff --git a/Documentation/git.txt b/Documentation/git.txt
index 1a6ba87823..bec6348dab 100644
--- a/Documentation/git.txt
+++ b/Documentation/git.txt
@@ -545,6 +545,16 @@ git so take care if using Cogito etc.
 	a GIT_DIR set on the command line or in the environment.
 	(Useful for excluding slow-loading network directories.)
 
+'GIT_DISCOVERY_ACROSS_FILESYSTEM'::
+	When run in a directory that does not have ".git" repository
+	directory, git tries to find such a directory in the parent
+	directories to find the top of the working tree, but by default it
+	does not cross filesystem boundaries.  This environment variable
+	can be set to true to tell git not to stop at filesystem
+	boundaries.  Like 'GIT_CEILING_DIRECTORIES', this will not affect
+	an explicit repository directory set via 'GIT_DIR' or on the
+	command line.
+
 git Commits
 ~~~~~~~~~~~
 'GIT_AUTHOR_NAME'::
diff --git a/cache.h b/cache.h
index 8270390c88..d00d2be320 100644
--- a/cache.h
+++ b/cache.h
@@ -954,6 +954,7 @@ extern int git_config_set_multivar(const char *, const char *, const char *, int
 extern int git_config_rename_section(const char *, const char *);
 extern const char *git_etc_gitconfig(void);
 extern int check_repository_format_version(const char *var, const char *value, void *cb);
+extern int git_env_bool(const char *, int);
 extern int git_config_system(void);
 extern int git_config_global(void);
 extern int config_error_nonbool(const char *);
diff --git a/config.c b/config.c
index aab53298a1..9b6b1df212 100644
--- a/config.c
+++ b/config.c
@@ -739,7 +739,7 @@ const char *git_etc_gitconfig(void)
 	return system_wide;
 }
 
-static int git_env_bool(const char *k, int def)
+int git_env_bool(const char *k, int def)
 {
 	const char *v = getenv(k);
 	return v ? git_config_bool(k, v) : def;
diff --git a/setup.c b/setup.c
index 5716d90b57..5a083fa77d 100644
--- a/setup.c
+++ b/setup.c
@@ -323,6 +323,8 @@ const char *setup_git_directory_gently(int *nongit_ok)
 	const char *gitdirenv;
 	const char *gitfile_dir;
 	int len, offset, ceil_offset, root_len;
+	int current_device = 0, one_filesystem = 1;
+	struct stat buf;
 
 	/*
 	 * Let's assume that we are in a git repository.
@@ -390,6 +392,12 @@ const char *setup_git_directory_gently(int *nongit_ok)
 	 *   etc.
 	 */
 	offset = len = strlen(cwd);
+	one_filesystem = !git_env_bool("GIT_DISCOVERY_ACROSS_FILESYSTEM", 0);
+	if (one_filesystem) {
+		if (stat(".", &buf))
+			die_errno("failed to stat '.'");
+		current_device = buf.st_dev;
+	}
 	for (;;) {
 		gitfile_dir = read_gitfile_gently(DEFAULT_GIT_DIR_ENVIRONMENT);
 		if (gitfile_dir) {
@@ -422,8 +430,27 @@ const char *setup_git_directory_gently(int *nongit_ok)
 			}
 			die("Not a git repository (or any of the parent directories): %s", DEFAULT_GIT_DIR_ENVIRONMENT);
 		}
-		if (chdir(".."))
+		if (one_filesystem) {
+			if (stat("..", &buf)) {
+				cwd[offset] = '\0';
+				die_errno("failed to stat '%s/..'", cwd);
+			}
+			if (buf.st_dev != current_device) {
+				if (nongit_ok) {
+					if (chdir(cwd))
+						die_errno("Cannot come back to cwd");
+					*nongit_ok = 1;
+					return NULL;
+				}
+				cwd[offset] = '\0';
+				die("Not a git repository (or any parent up to mount parent %s)\n"
+				"Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).", cwd);
+			}
+		}
+		if (chdir("..")) {
+			cwd[offset] = '\0';
 			die_errno("Cannot change to '%s/..'", cwd);
+		}
 	}
 
 	inside_git_dir = 0;