Re: [PATCH] ovl: make private mounts longterm

Al Viro
2020-05-22T16:08:18Z

On Fri, May 22, 2020 at 10:57:23AM +0200, Miklos Szeredi wrote:
> 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.

Umm...

1) Documentation/filesystems/porting - something along the lines
of "clone_private_mount() returns a longterm mount now, so the proper
destructor of its result is kern_unmount()"

2) the name kern_unmount_many() has an unfortunate clash with
fput_many(), with arguments that look similar and mean something
entirely different. How about kern_unmount_array()?

3)
> - 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);

That's _way_ too subtle. AFAICS, you rely upon ->upper_mnt == ->layers[0].mnt,
->layers[0].trap == NULL, without even mentioning that. And the hack you do
mention... Yecchhh... How many layers are possible, again?