From 6422a01e84f6dd1873b9e2ba86f408e2a20f32db Mon Sep 17 00:00:00 2001
From: Miklos Szeredi <mszeredi@redhat.com>
Date: Thu, 4 Jun 2020 10:48:19 +0200
Subject: [PATCH] ovl: make private mounts longterm
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>
---
Documentation/filesystems/porting.rst | 7 +++++++
fs/namespace.c | 16 ++++++++++++++++
fs/overlayfs/super.c | 7 ++++++-
include/linux/mount.h | 2 ++
4 files changed, 31 insertions(+), 1 deletion(-)
diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst
index 26c0939695736..867036aa90b83 100644
--- a/Documentation/filesystems/porting.rst
+++ b/Documentation/filesystems/porting.rst
@@ -858,3 +858,10 @@ be misspelled d_alloc_anon().
[should've been added in 2016] stale comment in finish_open() nonwithstanding,
failure exits in ->atomic_open() instances should *NOT* fput() the file,
no matter what. Everything is handled by the caller.
+
+---
+
+**mandatory**
+
+clone_private_mount() returns a longterm mount now, so the proper destructor of
+its result is kern_unmount() or kern_unmount_array().
diff --git a/fs/namespace.c b/fs/namespace.c
index 6d499ab254b71..7cd6424091657 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1937,6 +1937,9 @@ 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);
@@ -3863,6 +3866,19 @@ void kern_unmount(struct vfsmount *mnt)
}
EXPORT_SYMBOL(kern_unmount);
+void kern_unmount_array(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_array);
+
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 eb81d8760a6a0..8d8cd46e14827 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -211,6 +211,7 @@ static void ovl_destroy_inode(struct inode *inode)
static void ovl_free_fs(struct ovl_fs *ofs)
{
+ struct vfsmount **mounts;
unsigned i;
iput(ofs->workbasedir_trap);
@@ -224,10 +225,14 @@ static void ovl_free_fs(struct ovl_fs *ofs)
dput(ofs->workbasedir);
if (ofs->upperdir_locked)
ovl_inuse_unlock(ovl_upper_mnt(ofs)->mnt_root);
+
+ /* Hack! Reuse ofs->layers as a vfsmount array before freeing it */
+ mounts = (struct vfsmount **) ofs->layers;
for (i = 0; i < ofs->numlayer; i++) {
iput(ofs->layers[i].trap);
- mntput(ofs->layers[i].mnt);
+ mounts[i] = ofs->layers[i].mnt;
}
+ kern_unmount_array(mounts, ofs->numlayer);
kfree(ofs->layers);
for (i = 0; i < ofs->numfs; i++)
free_anon_bdev(ofs->fs[i].pseudo_dev);
diff --git a/include/linux/mount.h b/include/linux/mount.h
index 7edac8c7a9c1c..de657bd211fa6 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -111,4 +111,6 @@ extern unsigned int sysctl_mount_max;
extern bool path_is_mountpoint(const struct path *path);
+extern void kern_unmount_array(struct vfsmount *mnt[], unsigned int num);
+
#endif /* _LINUX_MOUNT_H */
--
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