Re: [PATCH v3 08/13] mm: add ability to take further action in vm_area_desc

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Sep 16, 2025 at 03:11:54PM +0100, Lorenzo Stoakes wrote:

> +/* What action should be taken after an .mmap_prepare call is complete? */
> +enum mmap_action_type {
> +	MMAP_NOTHING,		/* Mapping is complete, no further action. */
> +	MMAP_REMAP_PFN,		/* Remap PFN range. */

Seems like it would be a bit tider to include MMAP_IO_REMAP_PFN here
instead of having the is_io_remap bool.

> @@ -1155,15 +1155,18 @@ int __compat_vma_mmap_prepare(const struct file_operations *f_op,
>  		.vm_file = vma->vm_file,
>  		.vm_flags = vma->vm_flags,
>  		.page_prot = vma->vm_page_prot,
> +
> +		.action.type = MMAP_NOTHING, /* Default */
>  	};
>  	int err;
>  
>  	err = f_op->mmap_prepare(&desc);
>  	if (err)
>  		return err;
> -	set_vma_from_desc(vma, &desc);
>  
> -	return 0;
> +	mmap_action_prepare(&desc.action, &desc);
> +	set_vma_from_desc(vma, &desc);
> +	return mmap_action_complete(&desc.action, vma);
>  }
>  EXPORT_SYMBOL(__compat_vma_mmap_prepare);

A function called prepare that now calls complete has become a bit oddly named??

> +int mmap_action_complete(struct mmap_action *action,
> +			 struct vm_area_struct *vma)
> +{
> +	int err = 0;
> +
> +	switch (action->type) {
> +	case MMAP_NOTHING:
> +		break;
> +	case MMAP_REMAP_PFN:
> +		VM_WARN_ON_ONCE((vma->vm_flags & VM_REMAP_FLAGS) !=
> +				VM_REMAP_FLAGS);

This is checked in remap_pfn_range_complete() IIRC? Probably not
needed here as well then.

> +		if (action->remap.is_io_remap)
> +			err = io_remap_pfn_range_complete(vma, action->remap.start,
> +				action->remap.start_pfn, action->remap.size,
> +				action->remap.pgprot);
> +		else
> +			err = remap_pfn_range_complete(vma, action->remap.start,
> +				action->remap.start_pfn, action->remap.size,
> +				action->remap.pgprot);
> +		break;
> +	}
> +
> +	/*
> +	 * If an error occurs, unmap the VMA altogether and return an error. We
> +	 * only clear the newly allocated VMA, since this function is only
> +	 * invoked if we do NOT merge, so we only clean up the VMA we created.
> +	 */
> +	if (err) {
> +		const size_t len = vma_pages(vma) << PAGE_SHIFT;
> +
> +		do_munmap(current->mm, vma->vm_start, len, NULL);
> +
> +		if (action->error_hook) {
> +			/* We may want to filter the error. */
> +			err = action->error_hook(err);
> +
> +			/* The caller should not clear the error. */
> +			VM_WARN_ON_ONCE(!err);
> +		}
> +		return err;
> +	}
> +
> +	if (action->success_hook)
> +		err = action->success_hook(vma);
> +
> +	return err;

I would write this as

	if (action->success_hook)
		return action->success_hook(vma);

	return 0;

Just for emphasis this is the success path.

> +int mmap_action_complete(struct mmap_action *action,
> +			struct vm_area_struct *vma)
> +{
> +	int err = 0;
> +
> +	switch (action->type) {
> +	case MMAP_NOTHING:
> +		break;
> +	case MMAP_REMAP_PFN:
> +		WARN_ON_ONCE(1); /* nommu cannot handle this. */
> +
> +		break;
> +	}
> +
> +	/*
> +	 * If an error occurs, unmap the VMA altogether and return an error. We
> +	 * only clear the newly allocated VMA, since this function is only
> +	 * invoked if we do NOT merge, so we only clean up the VMA we created.
> +	 */
> +	if (err) {
> +		const size_t len = vma_pages(vma) << PAGE_SHIFT;
> +
> +		do_munmap(current->mm, vma->vm_start, len, NULL);
> +
> +		if (action->error_hook) {
> +			/* We may want to filter the error. */
> +			err = action->error_hook(err);
> +
> +			/* The caller should not clear the error. */
> +			VM_WARN_ON_ONCE(!err);
> +		}
> +		return err;
> +	}

err is never !0 here, so this should go to a later patch/series.

Also seems like this cleanup wants to be in a function that is not
protected by #ifdef nommu since the code is identical on both branches.

> +	if (action->success_hook)
> +		err = action->success_hook(vma);
> +
> +	return 0;

return err, though prefer to match above, and probably this sequence
should be pulled into the same shared function as above too.

Jason