Commit df820f8d authored by Miklos Szeredi's avatar Miklos Szeredi
Browse files

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: default avatarMiklos Szeredi <mszeredi@redhat.com>
parent b8e42a65

Documentation/filesystems/porting.rst

+7 −0
Original line number Diff line number Diff line
@@ -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().

fs/namespace.c

+16 −0
Original line number Diff line number Diff line
@@ -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_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));

fs/overlayfs/super.c

+6 −1
Original line number Diff line number Diff line
@@ -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);

include/linux/mount.h

+2 −0
Original line number Diff line number Diff line
@@ -109,4 +109,6 @@ 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 */