Message-ID: <wabzfghapygwy3fzexbplmasrdzttt3nsgpmoj4kr6g7ldstkg@tthpx7de6tqk>
Date: Wed, 17 Sep 2025 12:32:10 +0100
From: Pedro Falcato <pfalcato@...e.de>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
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Subject: Re: [PATCH v3 08/13] mm: add ability to take further action in
 vm_area_desc

On Tue, Sep 16, 2025 at 03:11:54PM +0100, Lorenzo Stoakes wrote:
> Some drivers/filesystems need to perform additional tasks after the VMA is
> set up.  This is typically in the form of pre-population.
> 
> The forms of pre-population most likely to be performed are a PFN remap
> or the insertion of normal folios and PFNs into a mixed map.
> 
> We start by implementing the PFN remap functionality, ensuring that we
> perform the appropriate actions at the appropriate time - that is setting
> flags at the point of .mmap_prepare, and performing the actual remap at the
> point at which the VMA is fully established.
> 
> This prevents the driver from doing anything too crazy with a VMA at any
> stage, and we retain complete control over how the mm functionality is
> applied.
> 
> Unfortunately callers still do often require some kind of custom action,
> so we add an optional success/error _hook to allow the caller to do
> something after the action has succeeded or failed.

Do we have any idea for rules regarding ->mmap_prepare() and ->*_hook()?
It feels spooky to e.g grab locks in mmap_prepare, and hold them across core
mmap(). And I guess it might be needed?

> 
> This is done at the point when the VMA has already been established, so
> the harm that can be done is limited.
> 
> The error hook can be used to filter errors if necessary.
> 
> If any error arises on these final actions, we simply unmap the VMA
> altogether.
> 
> Also update the stacked filesystem compatibility layer to utilise the
> action behaviour, and update the VMA tests accordingly.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
<snip>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 31b27086586d..aa1e2003f366 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -775,6 +775,49 @@ struct pfnmap_track_ctx {
>  };
>  #endif
>  
> +/* 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;
> +			bool is_io_remap;
> +		} 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);

Do we need two hooks? It might be more ergonomic to simply have a:

	int (*finish)(int err);


	int random_driver_finish(int err)
	{
		if (err)
			pr_err("ahhhhhhhhh\n");
		mutex_unlock(&big_lock);
		return err;
	}

It's also unclear to me if/why we need the capability to switch error codes,
but I might've missed some discussion on this.


-- 
Pedro

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