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ovl: make private mounts longterm

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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>
This commit is contained in:
Miklos Szeredi 2020-06-04 10:48:19 +02:00
parent b8e42a651b
commit df820f8de4
4 changed files with 31 additions and 1 deletions
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7
Documentation/filesystems/porting.rst
View File

@ -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().

16
fs/namespace.c
View File

@ -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));

7
fs/overlayfs/super.c
View File

@ -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);

2
include/linux/mount.h
View File

@ -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 */