Re: [PATCH v6 1/3] revocable: Add fops replacement

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Nov 06, 2025 at 11:27:10PM +0800, Tzung-Bi Shih wrote:
> +/*
> + * Recover the private_data to its original one.
> + */
> +static struct fops_replacement *_recover_private_data(struct file *filp)
> +{
> +	struct fops_replacement *fr = filp->private_data;
> +
> +	filp->private_data = fr->orig_private_data;
> +	return fr;
> +}
> +
> +/*
> + * Replace the private_data to fops_replacement.
> + */
> +static void _replace_private_data(struct fops_replacement *fr)
> +{
> +	fr->filp->private_data = fr;
> +}

This switching of private_data isn't reasonable, it breaks too much
stuff. I think I showed a better idea in my sketch.

I still think this is a bad use case of revocable, we don't need to
obfuscate very simple locks in *core* kernel code like this. I'd rather
see you propose this series without using it.

> +static int fs_revocable_release(struct inode *inode, struct file *filp)
> +{
> +	struct fops_replacement *fr = _recover_private_data(filp);
> +	int ret = 0;
> +	void *any;
> +
> +	filp->f_op = fr->orig_fops;
> +
> +	if (!fr->orig_fops->release)
> +		goto leave;
> +
> +	REVOCABLE_TRY_ACCESS_SCOPED(fr->rev, any) {
> +		if (!any) {
> +			ret = -ENODEV;
> +			goto leave;
> +		}
> +
> +		ret = fr->orig_fops->release(inode, filp);
> +	}

This probably doesn't work out, is likely to make a memory leak.
It will be hard for the owning driver to free its per-file memory
without access to release.

Jason