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crun/SOURCES/1859.patch

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From 1556c13f89f5db22911a4e771af9253a9b79e02c Mon Sep 17 00:00:00 2001
From: Sohan Kunkerkar <sohank2602@gmail.com>
Date: Thu, 28 Aug 2025 08:53:20 -0400
Subject: [PATCH 1/2] src/libcrun: limit tmpfs memory usage for masked paths
Replace "size=0k" (unlimited growth) with explicit block and inode limits
for tmpfs mounts used in masked directory paths. This prevents excessive
kernel memory consumption under high container density.
Signed-off-by: Sohan Kunkerkar <sohank2602@gmail.com>
---
src/libcrun/linux.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/src/libcrun/linux.c b/src/libcrun/linux.c
index 75120cea37..36ed40bb5b 100644
--- a/src/libcrun/linux.c
+++ b/src/libcrun/linux.c
@@ -1114,7 +1114,7 @@ do_masked_or_readonly_path (libcrun_container_t *container, const char *rel_path
return crun_make_error (err, errno, "cannot stat `%s`", rel_path);
if ((mode & S_IFMT) == S_IFDIR)
- ret = do_mount (container, "tmpfs", pathfd, rel_path, "tmpfs", MS_RDONLY, "size=0k", LABEL_MOUNT, err);
+ ret = do_mount (container, "tmpfs", pathfd, rel_path, "tmpfs", MS_RDONLY, "nr_blocks=1,nr_inodes=1", LABEL_MOUNT, err);
else
ret = do_mount (container, "/dev/null", pathfd, rel_path, NULL, MS_BIND | MS_RDONLY, NULL, LABEL_MOUNT, err);
if (UNLIKELY (ret < 0))
From 4004e5bed9ff52029a829131fbc16f9a877154b9 Mon Sep 17 00:00:00 2001
From: Sohan Kunkerkar <sohank2602@gmail.com>
Date: Tue, 26 Aug 2025 23:22:56 -0400
Subject: [PATCH 2/2] linux: optimize masked paths with shared empty directory
Optimize masked path handling by bind-mounting a shared empty directory
(via cached /proc/self/fd) instead of creating per-path tmpfs mounts.
This reduces kernel memory and mount syscall overhead under high container
density.
Signed-off-by: Sohan Kunkerkar <sohank2602@gmail.com>
---
src/libcrun/linux.c | 110 ++++++++++++++++++++++++++++++++++++++++++-
src/libcrun/status.c | 2 +-
src/libcrun/status.h | 1 +
3 files changed, 111 insertions(+), 2 deletions(-)
diff --git a/src/libcrun/linux.c b/src/libcrun/linux.c
index 36ed40bb5b..552715f729 100644
--- a/src/libcrun/linux.c
+++ b/src/libcrun/linux.c
@@ -21,6 +21,7 @@
#include <config.h>
#include "linux.h"
#include "utils.h"
+#include "status.h"
#include <string.h>
#include <sched.h>
#include <fcntl.h>
@@ -148,6 +149,12 @@ struct private_data_s
/* Used to save stdin, stdout, stderr during checkpointing to descriptors.json
* and needed during restore. */
char *external_descriptors;
+
+ /* Cached shared empty directory for masked paths optimization */
+ int maskdir_fd;
+ char *maskdir_proc_path;
+ bool maskdir_bind_failed;
+ bool maskdir_warned;
};
struct linux_namespace_s
@@ -164,6 +171,8 @@ cleanup_private_data (void *private_data)
if (p->rootfsfd >= 0)
TEMP_FAILURE_RETRY (close (p->rootfsfd));
+ if (p->maskdir_fd >= 0)
+ TEMP_FAILURE_RETRY (close (p->maskdir_fd));
if (p->mount_fds)
cleanup_close_mapp (&(p->mount_fds));
if (p->dev_fds)
@@ -173,6 +182,7 @@ cleanup_private_data (void *private_data)
free (p->host_notify_socket_path);
free (p->container_notify_socket_path);
free (p->external_descriptors);
+ free (p->maskdir_proc_path);
free (p);
}
@@ -185,6 +195,7 @@ get_private_data (struct libcrun_container_s *container)
container->private_data = p;
p->rootfsfd = -1;
p->notify_socket_tree_fd = -1;
+ p->maskdir_fd = -1;
container->cleanup_private_data = cleanup_private_data;
}
return container->private_data;
@@ -1058,6 +1069,103 @@ has_mount_for (libcrun_container_t *container, const char *destination)
return false;
}
+static void
+warn_tmpfs_fallback_once (struct private_data_s *private_data, const char *reason)
+{
+ if (! private_data->maskdir_warned)
+ {
+ libcrun_warning ("Falling back to tmpfs for masked dirs (reason: %s)", reason);
+ private_data->maskdir_warned = true;
+ }
+}
+
+/* Get or create the cached shared empty directory for masked paths optimization.
+ * Creates directory and FD once per container, caches /proc/self/fd path for fast mounting.
+ */
+static int
+get_shared_empty_dir_cached (libcrun_container_t *container, char **proc_fd_path, libcrun_error_t *err)
+{
+ struct private_data_s *private_data = get_private_data (container);
+ cleanup_close int fd = -1;
+ cleanup_free char *run_dir = NULL;
+ cleanup_free char *empty_dir_path = NULL;
+ int ret;
+
+ /* Fast path: return cached proc fd path if already set up */
+ if (private_data->maskdir_proc_path != NULL)
+ {
+ *proc_fd_path = private_data->maskdir_proc_path;
+ return 0;
+ }
+
+ /* Slow path: create directory and cache everything once */
+ ret = get_run_directory (&run_dir, container->context->state_root, err);
+ if (UNLIKELY (ret < 0))
+ return ret;
+
+ ret = append_paths (&empty_dir_path, err, run_dir, ".empty-directory", NULL);
+ if (UNLIKELY (ret < 0))
+ return ret;
+
+ /* Ensure the empty directory exists (once per container) */
+ ret = crun_ensure_directory (empty_dir_path, 0555, false, err);
+ if (UNLIKELY (ret < 0))
+ return ret;
+
+ /* Open directory and cache FD (once per container) */
+ fd = open (empty_dir_path, O_DIRECTORY | O_RDONLY | O_CLOEXEC);
+ if (fd < 0)
+ return crun_make_error (err, errno, "open directory `%s`", empty_dir_path);
+
+ /* Cache the /proc/self/fd path for fast mounting */
+ ret = xasprintf (&private_data->maskdir_proc_path, "/proc/self/fd/%d", fd);
+ if (UNLIKELY (ret < 0))
+ return crun_make_error (err, errno, "xasprintf failed");
+
+ private_data->maskdir_fd = fd;
+ fd = -1; /* Don't auto-close */
+
+ *proc_fd_path = private_data->maskdir_proc_path;
+ return 0;
+}
+
+static int
+mount_masked_dir (libcrun_container_t *container, int pathfd, const char *rel_path, libcrun_error_t *err)
+{
+ struct private_data_s *private_data = get_private_data (container);
+ char *proc_fd_path = NULL;
+ libcrun_error_t tmp_err = NULL;
+ int ret;
+
+ if (private_data->maskdir_bind_failed)
+ goto fallback_to_tmpfs;
+
+ /* Get cached /proc/self/fd path (fast after first call) */
+ ret = get_shared_empty_dir_cached (container, &proc_fd_path, &tmp_err);
+ if (ret < 0)
+ {
+ private_data->maskdir_bind_failed = true;
+ warn_tmpfs_fallback_once (private_data, tmp_err->msg);
+ crun_error_release (&tmp_err);
+ goto fallback_to_tmpfs;
+ }
+
+ ret = do_mount (container, proc_fd_path, pathfd, rel_path, NULL, MS_BIND | MS_RDONLY, NULL, LABEL_MOUNT, &tmp_err);
+ if (LIKELY (ret >= 0))
+ return ret;
+
+ /* Bind mount failed - mark as failed and fall back for all future mounts */
+ private_data->maskdir_bind_failed = true;
+ libcrun_warning ("bind mount failed for %s to %s: %s, falling back to tmpfs",
+ proc_fd_path, rel_path, tmp_err->msg);
+ warn_tmpfs_fallback_once (private_data, tmp_err->msg);
+ crun_error_release (&tmp_err);
+
+fallback_to_tmpfs:
+ libcrun_debug ("using tmpfs fallback for %s", rel_path);
+ return ret = do_mount (container, "tmpfs", pathfd, rel_path, "tmpfs", MS_RDONLY, "nr_blocks=1,nr_inodes=1", LABEL_MOUNT, err);
+}
+
static int
do_masked_or_readonly_path (libcrun_container_t *container, const char *rel_path, bool readonly, bool keep_flags,
libcrun_error_t *err)
@@ -1114,7 +1222,7 @@ do_masked_or_readonly_path (libcrun_container_t *container, const char *rel_path
return crun_make_error (err, errno, "cannot stat `%s`", rel_path);
if ((mode & S_IFMT) == S_IFDIR)
- ret = do_mount (container, "tmpfs", pathfd, rel_path, "tmpfs", MS_RDONLY, "nr_blocks=1,nr_inodes=1", LABEL_MOUNT, err);
+ ret = mount_masked_dir (container, pathfd, rel_path, err);
else
ret = do_mount (container, "/dev/null", pathfd, rel_path, NULL, MS_BIND | MS_RDONLY, NULL, LABEL_MOUNT, err);
if (UNLIKELY (ret < 0))
diff --git a/src/libcrun/status.c b/src/libcrun/status.c
index 714a31adc7..c786ef6ea9 100644
--- a/src/libcrun/status.c
+++ b/src/libcrun/status.c
@@ -55,7 +55,7 @@ validate_id (const char *id, libcrun_error_t *err)
return 0;
}
-static int
+int
get_run_directory (char **out, const char *state_root, libcrun_error_t *err)
{
int ret;
diff --git a/src/libcrun/status.h b/src/libcrun/status.h
index cd6c0ced16..72a94348a5 100644
--- a/src/libcrun/status.h
+++ b/src/libcrun/status.h
@@ -65,6 +65,7 @@ int libcrun_status_create_exec_fifo (const char *state_root, const char *id, lib
int libcrun_status_write_exec_fifo (const char *state_root, const char *id, libcrun_error_t *err);
int libcrun_status_has_read_exec_fifo (const char *state_root, const char *id, libcrun_error_t *err);
int libcrun_check_pid_valid (libcrun_container_status_t *status, libcrun_error_t *err);
+int get_run_directory (char **out, const char *state_root, libcrun_error_t *err);
static inline void
libcrun_free_container_listp (void *p)
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