[PATCH 19/38] ipc: Convert mqueue fs to fs_context [ver #10]

David Howells
2018-07-27T17:33:37Z

Convert the mqueue filesystem to use the filesystem context stuff.

Notes:

(1) The relevant ipc namespace is selected in when the context is
initialised (and it defaults to the current task's ipc namespace).
The caller can override this before calling vfs_get_tree().

(2) Rather than simply calling kern_mount_data(), mq_init_ns() and
mq_internal_mount() create a context, adjust it and then do the rest
of the mount procedure.

(3) The lazy mqueue mounting on creation of a new namespace is retained
from a previous patch, but the avoidance of sget() if no superblock
yet exists is reverted and the superblock is again keyed on the
namespace pointer.

Yes, there was a performance gain in not searching the superblock
hash, but it's only paid once per ipc namespace - and only if someone
uses mqueue within that namespace, so I'm not sure it's worth it,
especially as calling sget() allows avoidance of recursion.

Signed-off-by: David Howells <dhowells@redhat.com>
---

ipc/mqueue.c | 121 +++++++++++++++++++++++++++++++++++++++++++++++-----------
1 file changed, 99 insertions(+), 22 deletions(-)

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 4671d215cb84..0f102210f89e 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -18,6 +18,7 @@
#include <linux/pagemap.h>
#include <linux/file.h>
#include <linux/mount.h>
+#include <linux/fs_context.h>
#include <linux/namei.h>
#include <linux/sysctl.h>
#include <linux/poll.h>
@@ -42,6 +43,10 @@
#include <net/sock.h>
#include "util.h"

+struct mqueue_fs_context {
+ struct ipc_namespace *ipc_ns;
+};
+
#define MQUEUE_MAGIC 0x19800202
#define DIRENT_SIZE 20
#define FILENT_SIZE 80
@@ -87,9 +92,11 @@ struct mqueue_inode_info {
unsigned long qsize; /* size of queue in memory (sum of all msgs) */
};

+static struct file_system_type mqueue_fs_type;
static const struct inode_operations mqueue_dir_inode_operations;
static const struct file_operations mqueue_file_operations;
static const struct super_operations mqueue_super_ops;
+static const struct fs_context_operations mqueue_fs_context_ops;
static void remove_notification(struct mqueue_inode_info *info);

static struct kmem_cache *mqueue_inode_cachep;
@@ -322,7 +329,7 @@ static struct inode *mqueue_get_inode(struct super_block *sb,
return ERR_PTR(ret);
}

-static int mqueue_fill_super(struct super_block *sb, void *data, size_t data_size, int silent)
+static int mqueue_fill_super(struct super_block *sb, struct fs_context *fc)
{
struct inode *inode;
struct ipc_namespace *ns = sb->s_fs_info;
@@ -343,19 +350,84 @@ static int mqueue_fill_super(struct super_block *sb, void *data, size_t data_siz
return 0;
}

-static struct dentry *mqueue_mount(struct file_system_type *fs_type,
- int flags, const char *dev_name,
- void *data, size_t data_size)
+static int mqueue_get_tree(struct fs_context *fc)
{
- struct ipc_namespace *ns;
- if (flags & SB_KERNMOUNT) {
- ns = data;
- data = NULL;
- } else {
- ns = current->nsproxy->ipc_ns;
+ struct mqueue_fs_context *ctx = fc->fs_private;
+
+ /* As a shortcut, if the namespace already has a superblock created,
+ * use the root from that directly rather than invoking sget() again.
+ */
+ spin_lock(&mq_lock);
+ if (ctx->ipc_ns->mq_mnt) {
+ fc->root = dget(ctx->ipc_ns->mq_mnt->mnt_sb->s_root);
+ atomic_inc(&fc->root->d_sb->s_active);
}
- return mount_ns(fs_type, flags, data, data_size, ns, ns->user_ns,
- mqueue_fill_super);
+ spin_unlock(&mq_lock);
+ if (fc->root) {
+ down_write(&fc->root->d_sb->s_umount);
+ return 0;
+ }
+
+ fc->s_fs_info = ctx->ipc_ns;
+ return vfs_get_super(fc, vfs_get_keyed_super, mqueue_fill_super);
+}
+
+static void mqueue_fs_context_free(struct fs_context *fc)
+{
+ struct mqueue_fs_context *ctx = fc->fs_private;
+
+ if (ctx->ipc_ns)
+ put_ipc_ns(ctx->ipc_ns);
+ kfree(ctx);
+}
+
+static int mqueue_init_fs_context(struct fs_context *fc,
+ struct dentry *reference)
+{
+ struct mqueue_fs_context *ctx;
+
+ ctx = kzalloc(sizeof(struct mqueue_fs_context), GFP_KERNEL);
+ if (!ctx)
+ return -ENOMEM;
+
+ ctx->ipc_ns = get_ipc_ns(current->nsproxy->ipc_ns);
+ fc->fs_private = ctx;
+ fc->ops = &mqueue_fs_context_ops;
+ return 0;
+}
+
+static struct vfsmount *mq_create_mount(struct ipc_namespace *ns)
+{
+ struct mqueue_fs_context *ctx;
+ struct fs_context *fc;
+ struct vfsmount *mnt;
+ int ret;
+
+ fc = vfs_new_fs_context(&mqueue_fs_type, NULL, 0,
+ FS_CONTEXT_FOR_KERNEL_MOUNT);
+ if (IS_ERR(fc))
+ return ERR_CAST(fc);
+
+ ctx = fc->fs_private;
+ put_ipc_ns(ctx->ipc_ns);
+ ctx->ipc_ns = get_ipc_ns(ns);
+
+ ret = vfs_get_tree(fc);
+ if (ret < 0)
+ goto err_fc;
+
+ mnt = vfs_create_mount(fc, 0);
+ if (IS_ERR(mnt)) {
+ ret = PTR_ERR(mnt);
+ goto err_fc;
+ }
+
+ put_fs_context(fc);
+ return mnt;
+
+err_fc:
+ put_fs_context(fc);
+ return ERR_PTR(ret);
}

static void init_once(void *foo)
@@ -1523,15 +1595,22 @@ static const struct super_operations mqueue_super_ops = {
.statfs = simple_statfs,
};

+static const struct fs_context_operations mqueue_fs_context_ops = {
+ .free = mqueue_fs_context_free,
+ .get_tree = mqueue_get_tree,
+};
+
static struct file_system_type mqueue_fs_type = {
- .name = "mqueue",
- .mount = mqueue_mount,
- .kill_sb = kill_litter_super,
- .fs_flags = FS_USERNS_MOUNT,
+ .name = "mqueue",
+ .init_fs_context = mqueue_init_fs_context,
+ .kill_sb = kill_litter_super,
+ .fs_flags = FS_USERNS_MOUNT,
};

int mq_init_ns(struct ipc_namespace *ns)
{
+ struct vfsmount *m;
+
ns->mq_queues_count = 0;
ns->mq_queues_max = DFLT_QUEUESMAX;
ns->mq_msg_max = DFLT_MSGMAX;
@@ -1539,12 +1618,10 @@ int mq_init_ns(struct ipc_namespace *ns)
ns->mq_msg_default = DFLT_MSG;
ns->mq_msgsize_default = DFLT_MSGSIZE;

- ns->mq_mnt = kern_mount_data(&mqueue_fs_type, ns, 0);
- if (IS_ERR(ns->mq_mnt)) {
- int err = PTR_ERR(ns->mq_mnt);
- ns->mq_mnt = NULL;
- return err;
- }
+ m = mq_create_mount(&init_ipc_ns);
+ if (IS_ERR(m))
+ return PTR_ERR(m);
+ ns->mq_mnt = m;
return 0;
}