core: rework how we match mount units against each other

Previously to automatically create dependencies between mount units we
matched every mount unit agains all others resulting in O(n^2)
complexity. On setups with large amounts of mount units this might make
things slow.

This change replaces the matching code to use a hashtable that is keyed
by a path prefix, and points to a set of units that require that path to
be around. When a new mount unit is installed it is hence sufficient to
simply look up this set of units via its own file system paths to know
which units to order after itself.

This patch also changes all unit types to only create automatic mount
dependencies via the RequiresMountsFor= logic, and this is exposed to
the outside to make things more transparent.

With this change we still have some O(n) complexities in place when
handling mounts, but that's currently unavoidable due to kernel APIs,
and still substantially better than O(n^2) as before.

https://bugs.freedesktop.org/show_bug.cgi?id=69740

1 files changed, 66 insertions, 189 deletions

diff --git a/src/core/mount.c b/src/core/mount.c
index 78c5c1e6d6..3d46557fb1 100644
--- a/src/core/mount.c
+++ b/src/core/mount.c
@@ -157,138 +157,58 @@ _pure_ static MountParameters* get_mount_parameters(Mount *m) {
 }
 
 static int mount_add_mount_links(Mount *m) {
-        Unit *other;
-        int r;
+        _cleanup_free_ char *parent = NULL;
         MountParameters *pm;
-
-        assert(m);
-
-        pm = get_mount_parameters_fragment(m);
-
-        /* Adds in links to other mount points that might lie below or
-         * above us in the hierarchy */
-
-        LIST_FOREACH(units_by_type, other, UNIT(m)->manager->units_by_type[UNIT_MOUNT]) {
-                Mount *n = MOUNT(other);
-                MountParameters *pn;
-
-                if (n == m)
-                        continue;
-
-                if (UNIT(n)->load_state != UNIT_LOADED)
-                        continue;
-
-                pn = get_mount_parameters_fragment(n);
-
-                if (path_startswith(m->where, n->where)) {
-
-                        if ((r = unit_add_dependency(UNIT(m), UNIT_AFTER, UNIT(n), true)) < 0)
-                                return r;
-
-                        if (pn)
-                                if ((r = unit_add_dependency(UNIT(m), UNIT_REQUIRES, UNIT(n), true)) < 0)
-                                        return r;
-
-                } else if (path_startswith(n->where, m->where)) {
-
-                        if ((r = unit_add_dependency(UNIT(n), UNIT_AFTER, UNIT(m), true)) < 0)
-                                return r;
-
-                        if (pm)
-                                if ((r = unit_add_dependency(UNIT(n), UNIT_REQUIRES, UNIT(m), true)) < 0)
-                                        return r;
-
-                } else if (pm && pm->what && path_startswith(pm->what, n->where)) {
-
-                        if ((r = unit_add_dependency(UNIT(m), UNIT_AFTER, UNIT(n), true)) < 0)
-                                return r;
-
-                        if ((r = unit_add_dependency(UNIT(m), UNIT_REQUIRES, UNIT(n), true)) < 0)
-                                return r;
-
-                } else if (pn && pn->what && path_startswith(pn->what, m->where)) {
-
-                        if ((r = unit_add_dependency(UNIT(n), UNIT_AFTER, UNIT(m), true)) < 0)
-                                return r;
-
-                        if ((r = unit_add_dependency(UNIT(n), UNIT_REQUIRES, UNIT(m), true)) < 0)
-                                return r;
-                }
-        }
-
-        return 0;
-}
-
-static int mount_add_swap_links(Mount *m) {
         Unit *other;
+        Iterator i;
+        Set *s;
         int r;
 
         assert(m);
 
-        LIST_FOREACH(units_by_type, other, UNIT(m)->manager->units_by_type[UNIT_SWAP]) {
-                r = swap_add_one_mount_link(SWAP(other), m);
+        if (!path_equal(m->where, "/")) {
+                /* Adds in links to other mount points that might lie further
+                 * up in the hierarchy */
+                r = path_get_parent(m->where, &parent);
                 if (r < 0)
                         return r;
-        }
 
-        return 0;
-}
-
-static int mount_add_path_links(Mount *m) {
-        Unit *other;
-        int r;
-
-        assert(m);
-
-        LIST_FOREACH(units_by_type, other, UNIT(m)->manager->units_by_type[UNIT_PATH]) {
-                r = path_add_one_mount_link(PATH(other), m);
+                r = unit_require_mounts_for(UNIT(m), parent);
                 if (r < 0)
                         return r;
         }
 
-        return 0;
-}
-
-static int mount_add_automount_links(Mount *m) {
-        Unit *other;
-        int r;
-
-        assert(m);
-
-        LIST_FOREACH(units_by_type, other, UNIT(m)->manager->units_by_type[UNIT_AUTOMOUNT]) {
-                r = automount_add_one_mount_link(AUTOMOUNT(other), m);
+        /* Adds in links to other mount points that might be needed
+         * for the source path (if this is a bind mount) to be
+         * available. */
+        pm = get_mount_parameters_fragment(m);
+        if (pm && path_is_absolute(pm->what)) {
+                r = unit_require_mounts_for(UNIT(m), pm->what);
                 if (r < 0)
                         return r;
         }
 
-        return 0;
-}
+        /* Adds in links to other units that use this path or paths
+         * further down in the hierarchy */
+        s = manager_get_units_requiring_mounts_for(UNIT(m)->manager, m->where);
+        SET_FOREACH(other, s, i) {
 
-static int mount_add_socket_links(Mount *m) {
-        Unit *other;
-        int r;
+                if (other->load_state != UNIT_LOADED)
+                        continue;
 
-        assert(m);
+                if (other == UNIT(m))
+                        continue;
 
-        LIST_FOREACH(units_by_type, other, UNIT(m)->manager->units_by_type[UNIT_SOCKET]) {
-                r = socket_add_one_mount_link(SOCKET(other), m);
+                r = unit_add_dependency(other, UNIT_AFTER, UNIT(m), true);
                 if (r < 0)
                         return r;
-        }
 
-        return 0;
-}
-
-static int mount_add_requires_mounts_links(Mount *m) {
-        Unit *other;
-        int r;
-
-        assert(m);
-
-        LIST_FOREACH(has_requires_mounts_for, other, UNIT(m)->manager->has_requires_mounts_for) {
-                r = unit_add_one_mount_link(other, m);
-                if (r < 0)
-                        return r;
+                if (UNIT(m)->fragment_path) {
+                        /* If we have fragment configuration, then make this dependency required */
+                        r = unit_add_dependency(other, UNIT_REQUIRES, UNIT(m), true);
+                        if (r < 0)
+                                return r;
+                }
         }
 
         return 0;
@@ -567,8 +487,9 @@ static int mount_fix_timeouts(Mount *m) {
 }
 
 static int mount_verify(Mount *m) {
+        _cleanup_free_ char *e = NULL;
         bool b;
-        char *e;
+
         assert(m);
 
         if (UNIT(m)->load_state != UNIT_LOADED)
@@ -577,12 +498,11 @@ static int mount_verify(Mount *m) {
         if (!m->from_fragment && !m->from_proc_self_mountinfo)
                 return -ENOENT;
 
-        if (!(e = unit_name_from_path(m->where, ".mount")))
+        e = unit_name_from_path(m->where, ".mount");
+        if (!e)
                 return -ENOMEM;
 
         b = unit_has_name(UNIT(m), e);
-        free(e);
-
         if (!b) {
                 log_error_unit(UNIT(m)->id,
                                "%s's Where setting doesn't match unit name. Refusing.",
@@ -646,26 +566,6 @@ static int mount_add_extras(Mount *m) {
         if (r < 0)
                 return r;
 
-        r = mount_add_socket_links(m);
-        if (r < 0)
-                return r;
-
-        r = mount_add_swap_links(m);
-        if (r < 0)
-                return r;
-
-        r = mount_add_path_links(m);
-        if (r < 0)
-                return r;
-
-        r = mount_add_requires_mounts_links(m);
-        if (r < 0)
-                return r;
-
-        r = mount_add_automount_links(m);
-        if (r < 0)
-                return r;
-
         r = mount_add_quota_links(m);
         if (r < 0)
                 return r;
@@ -1650,79 +1550,56 @@ fail:
 static int mount_load_proc_self_mountinfo(Manager *m, bool set_flags) {
         int r = 0;
         unsigned i;
-        char *device, *path, *options, *options2, *fstype, *d, *p, *o;
 
         assert(m);
 
         rewind(m->proc_self_mountinfo);
 
         for (i = 1;; i++) {
+                _cleanup_free_ char *device = NULL, *path = NULL, *options = NULL, *options2 = NULL, *fstype = NULL, *d = NULL, *p = NULL, *o = NULL;
                 int k;
 
-                device = path = options = options2 = fstype = d = p = o = NULL;
-
-                if ((k = fscanf(m->proc_self_mountinfo,
-                                "%*s "       /* (1) mount id */
-                                "%*s "       /* (2) parent id */
-                                "%*s "       /* (3) major:minor */
-                                "%*s "       /* (4) root */
-                                "%ms "       /* (5) mount point */
-                                "%ms"        /* (6) mount options */
-                                "%*[^-]"     /* (7) optional fields */
-                                "- "         /* (8) separator */
-                                "%ms "       /* (9) file system type */
-                                "%ms"        /* (10) mount source */
-                                "%ms"        /* (11) mount options 2 */
-                                "%*[^\n]",   /* some rubbish at the end */
-                                &path,
-                                &options,
-                                &fstype,
-                                &device,
-                                &options2)) != 5) {
-
-                        if (k == EOF)
-                                break;
-
+                k = fscanf(m->proc_self_mountinfo,
+                           "%*s "       /* (1) mount id */
+                           "%*s "       /* (2) parent id */
+                           "%*s "       /* (3) major:minor */
+                           "%*s "       /* (4) root */
+                           "%ms "       /* (5) mount point */
+                           "%ms"        /* (6) mount options */
+                           "%*[^-]"     /* (7) optional fields */
+                           "- "         /* (8) separator */
+                           "%ms "       /* (9) file system type */
+                           "%ms"        /* (10) mount source */
+                           "%ms"        /* (11) mount options 2 */
+                           "%*[^\n]",   /* some rubbish at the end */
+                           &path,
+                           &options,
+                           &fstype,
+                           &device,
+                           &options2);
+
+                if (k == EOF)
+                        break;
+
+                if (k != 5) {
                         log_warning("Failed to parse /proc/self/mountinfo:%u.", i);
-                        goto clean_up;
+                        continue;
                 }
 
                 o = strjoin(options, ",", options2, NULL);
-                if (!o) {
-                        r = -ENOMEM;
-                        goto finish;
-                }
+                if (!o)
+                        return log_oom();
 
-                if (!(d = cunescape(device)) ||
-                    !(p = cunescape(path))) {
-                        r = -ENOMEM;
-                        goto finish;
-                }
+                d = cunescape(device);
+                p = cunescape(path);
+                if (!d || !p)
+                        return log_oom();
 
-                if ((k = mount_add_one(m, d, p, o, fstype, 0, set_flags)) < 0)
+                k = mount_add_one(m, d, p, o, fstype, 0, set_flags);
+                if (k < 0)
                         r = k;
-
-clean_up:
-                free(device);
-                free(path);
-                free(options);
-                free(options2);
-                free(fstype);
-                free(d);
-                free(p);
-                free(o);
         }
 
-finish:
-        free(device);
-        free(path);
-        free(options);
-        free(options2);
-        free(fstype);
-        free(d);
-        free(p);
-        free(o);
-
         return r;
 }