[PATCH] ovl: make private mounts longterm

Miklos Szeredi
2020-05-22T08:57:33Z

Overlayfs is using clone_private_mount() to create internal mounts for
underlying layers. These are used for operations requiring a path, such as
dentry_open().

Since these private mounts are not in any namespace they are treated as
short term, "detached" mounts and mntput() involves taking the global
mount_lock, which can result in serious cacheline pingpong.

Make these private mounts longterm instead, which trade the penalty on
mntput() for a slightly longer shutdown time due to an added RCU grace
period when putting these mounts.

Introduce a new helper kern_unmount_many() that can take care of multiple
longterm mounts with a single RCU grace period.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
fs/namespace.c | 16 ++++++++++++++++
fs/overlayfs/super.c | 19 ++++++++++++++-----
include/linux/mount.h | 2 ++
3 files changed, 32 insertions(+), 5 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index a28e4db075ed..5d16d87b6b8b 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1879,6 +1879,9 @@ struct vfsmount *clone_private_mount(const struct path *path)
if (IS_ERR(new_mnt))
return ERR_CAST(new_mnt);

+ /* Longterm mount to be removed by kern_unmount*() */
+ new_mnt->mnt_ns = MNT_NS_INTERNAL;
+
return &new_mnt->mnt;
}
EXPORT_SYMBOL_GPL(clone_private_mount);
@@ -3804,6 +3807,19 @@ void kern_unmount(struct vfsmount *mnt)
}
EXPORT_SYMBOL(kern_unmount);

+void kern_unmount_many(struct vfsmount *mnt[], unsigned int num)
+{
+ unsigned int i;
+
+ for (i = 0; i < num; i++)
+ if (mnt[i])
+ real_mount(mnt[i])->mnt_ns = NULL;
+ synchronize_rcu_expedited();
+ for (i = 0; i < num; i++)
+ mntput(mnt[i]);
+}
+EXPORT_SYMBOL(kern_unmount_many);
+
bool our_mnt(struct vfsmount *mnt)
{
return check_mnt(real_mount(mnt));
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 60dfb27bc12b..a938dd2521b2 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -225,12 +225,21 @@ static void ovl_free_fs(struct ovl_fs *ofs)
dput(ofs->workbasedir);
if (ofs->upperdir_locked)
ovl_inuse_unlock(ofs->upper_mnt->mnt_root);
- mntput(ofs->upper_mnt);
- for (i = 1; i < ofs->numlayer; i++) {
- iput(ofs->layers[i].trap);
- mntput(ofs->layers[i].mnt);
+
+ if (!ofs->layers) {
+ /* Deal with partial setup */
+ kern_unmount(ofs->upper_mnt);
+ } else {
+ /* Hack! Reuse ofs->layers as a mounts array */
+ struct vfsmount **mounts = (struct vfsmount **) ofs->layers;
+
+ for (i = 0; i < ofs->numlayer; i++) {
+ iput(ofs->layers[i].trap);
+ mounts[i] = ofs->layers[i].mnt;
+ }
+ kern_unmount_many(mounts, ofs->numlayer);
+ kfree(ofs->layers);
}
- kfree(ofs->layers);
for (i = 0; i < ofs->numfs; i++)
free_anon_bdev(ofs->fs[i].pseudo_dev);
kfree(ofs->fs);
diff --git a/include/linux/mount.h b/include/linux/mount.h
index bf8cc4108b8f..e3e994bfcecb 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -109,4 +109,6 @@ extern unsigned int sysctl_mount_max;

extern bool path_is_mountpoint(const struct path *path);

+extern void kern_unmount_many(struct vfsmount *mnt[], unsigned int num);
+
#endif /* _LINUX_MOUNT_H */
--
2.21.1