proc: Fix swap handling for cgroups v2 (can_use_swap)

On cgroups v2, there are no swap current/max files at the cgroup root, so
can_use_swap must look lower in the hierarchy to determine if swap accounting
is enabled. To also account for memory accounting being turned off at some
level, walk the hierarchy upwards from lxcfs' own cgroup.

Signed-off-by: Alex Hudspith <[email protected]>

src/bindings.c

@@ -866,6 +866,7 @@ static void __attribute__((constructor)) lxcfs_init(void)
 {
 	__do_close int init_ns = -EBADF, root_fd = -EBADF,
 				  pidfd = -EBADF;
+	__do_free char *cgroup = NULL;
 	int i = 0;
 	pid_t pid;
 	struct hierarchy *hierarchy;
@@ -920,7 +921,8 @@ static void __attribute__((constructor)) lxcfs_init(void)
 		lxcfs_info("Kernel supports pidfds");
 	}
 
-	can_use_swap = cgroup_ops->can_use_swap(cgroup_ops);
+	cgroup = get_pid_cgroup(pid, "memory");
+	can_use_swap = cgroup_ops->can_use_swap(cgroup_ops, cgroup);
 	if (can_use_swap)
 		lxcfs_info("Kernel supports swap accounting");
 	else

src/cgroups/cgfsng.c

@@ -631,34 +631,25 @@ static int cgfsng_get_memory_slabinfo_fd(struct cgroup_ops *ops, const char *cgr
 	return openat(h->fd, path, O_RDONLY | O_CLOEXEC | O_NOFOLLOW);
 }
 
-static bool cgfsng_can_use_swap(struct cgroup_ops *ops)
+static bool cgfsng_can_use_swap(struct cgroup_ops *ops, const char *cgroup)
 {
-	bool has_swap = false;
+	__do_free char *cgroup_rel = NULL, *junk_value = NULL;
+	const char *file;
 	struct hierarchy *h;
 
 	h = ops->get_hierarchy(ops, "memory");
 	if (!h)
 		return false;
 
-	if (is_unified_hierarchy(h)) {
-		if (faccessat(h->fd, "memory.swap.max", F_OK, 0))
-			return false;
-
-		if (faccessat(h->fd, "memory.swap.current", F_OK, 0))
-			return false;
-
-		has_swap = true;
-	} else {
-		if (faccessat(h->fd, "memory.memsw.limit_in_bytes", F_OK, 0))
-			return false;
-
-		if (faccessat(h->fd, "memory.memsw.usage_in_bytes", F_OK, 0))
-			return false;
-
-		has_swap = true;
-	}
-
-	return has_swap;
+	cgroup_rel = must_make_path_relative(cgroup, NULL);
+	file = is_unified_hierarchy(h) ? "memory.swap.current" : "memory.memsw.usage_in_bytes";
+	/* For v2, we need to look at the lower levels of the hierarchy because
+	 * no 'memory.swap.current' file exists at the root. We must search
+	 * upwards in the hierarchy in case memory accounting is disabled via
+	 * cgroup.subtree_control for the given cgroup itself.
+	 */
+	int ret = cgroup_walkup_to_root(ops->cgroup2_root_fd, h->fd, cgroup_rel, file, &junk_value);
+	return ret == 0;
 }
 
 static int cgfsng_get_memory_stats(struct cgroup_ops *ops, const char *cgroup,

src/cgroups/cgroup.h

@@ -148,7 +148,7 @@ struct cgroup_ops {
 				   char **value);
 	int (*get_memory_slabinfo_fd)(struct cgroup_ops *ops,
 				      const char *cgroup);
-	bool (*can_use_swap)(struct cgroup_ops *ops);
+	bool (*can_use_swap)(struct cgroup_ops *ops, const char *cgroup);
 
 	/* cpuset */
 	int (*get_cpuset_cpus)(struct cgroup_ops *ops, const char *cgroup,

src/proc_fuse.c

@@ -459,11 +459,13 @@ static int proc_swaps_read(char *buf, size_t size, off_t offset,
 	}
 
 	if (wants_swap) {
-		/* The total amount of swap is always reported to be the
+		/* For cgroups v1, the total amount of swap is always reported to be the
 		   lesser of the RAM+SWAP limit or the SWAP device size.
 		   This is because the kernel can swap as much as it
 		   wants and not only up to swtotal. */
-		swtotal = memlimit / 1024 + swtotal;
+		if (!liblxcfs_memory_is_cgroupv2())
+			swtotal = memlimit / 1024 + swtotal;
+
 		if (hostswtotal < swtotal) {
 			swtotal = hostswtotal;
 		}
@@ -1359,11 +1361,10 @@ static int proc_meminfo_read(char *buf, size_t size, off_t offset,
 
 				sscanf(line + STRLITERALLEN("SwapTotal:"), "%" PRIu64, &hostswtotal);
 
-				/* The total amount of swap is always reported to be the
+				/* In cgroups v1, the total amount of swap is always reported to be the
 				   lesser of the RAM+SWAP limit or the SWAP device size.
 				   This is because the kernel can swap as much as it
 				   wants and not only up to swtotal. */
-
 				if (!liblxcfs_memory_is_cgroupv2())
 					swtotal += memlimit;