linux-kernel - Re: [PATCH v4 09/14] mm: add ability to take further action in vm_area

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Message-ID: <20250917213737.GH1391379@nvidia.com>
Date: Wed, 17 Sep 2025 18:37:37 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>,
	Matthew Wilcox <willy@...radead.org>, Guo Ren <guoren@...nel.org>,
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Subject: Re: [PATCH v4 09/14] mm: add ability to take further action in
 vm_area_desc

On Wed, Sep 17, 2025 at 08:11:11PM +0100, Lorenzo Stoakes wrote:
> +static int mmap_action_finish(struct mmap_action *action,
> +		const struct vm_area_struct *vma, int err)
> +{
> +	/*
> +	 * 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)
> +		return action->success_hook(vma);

I thought you were going to use a single hook function as was
suggested?

return action->finish_hook(vma, err);

> +int mmap_action_complete(struct mmap_action *action,
> +			struct vm_area_struct *vma)
> +{
> +	switch (action->type) {
> +	case MMAP_NOTHING:
> +		break;
> +	case MMAP_REMAP_PFN:
> +	case MMAP_IO_REMAP_PFN:
> +		WARN_ON_ONCE(1); /* nommu cannot handle this. */

This should be:

     if (WARN_ON_ONCE(true))
         err = -EINVAL

To abort the thing and try to recover.

> diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
> index 07167446dcf4..22ed38e8714e 100644
> --- a/tools/testing/vma/vma_internal.h
> +++ b/tools/testing/vma/vma_internal.h
> @@ -274,6 +274,49 @@ struct mm_struct {
>  
>  struct vm_area_struct;
>  
> +
> +/* 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. */
> +};
> +
> +/*
> + * Describes an action an mmap_prepare hook can instruct to be taken to complete
> + * the mapping of a VMA. Specified in vm_area_desc.
> + */
> +struct mmap_action {
> +	union {
> +		/* Remap range. */
> +		struct {
> +			unsigned long start;
> +			unsigned long start_pfn;
> +			unsigned long size;
> +			pgprot_t pgprot;
> +		} remap;
> +	};
> +	enum mmap_action_type type;
> +
> +	/*
> +	 * If specified, this hook is invoked after the selected action has been
> +	 * successfully completed. Note that the VMA write lock still held.
> +	 *
> +	 * The absolute minimum ought to be done here.
> +	 *
> +	 * Returns 0 on success, or an error code.
> +	 */
> +	int (*success_hook)(const struct vm_area_struct *vma);
> +
> +	/*
> +	 * If specified, this hook is invoked when an error occurred when
> +	 * attempting the selection action.
> +	 *
> +	 * The hook can return an error code in order to filter the error, but
> +	 * it is not valid to clear the error here.
> +	 */
> +	int (*error_hook)(int err);
> +};

I didn't try to understand what vma_internal.h is for, but should this
block be an exact copy of the normal one? ie MMAP_IO_REMAP_PFN is missing?

Jason