Re: [PATCH 1/6] mm: Add a vm_special_mapping .fault method

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 10 Dec 2015 19:21:42 -0800 Andy Lutomirski <luto@kernel.org> wrote:

> From: Andy Lutomirski <luto@amacapital.net>
> 
> Requiring special mappings to give a list of struct pages is
> inflexible: it prevents sane use of IO memory in a special mapping,
> it's inefficient (it requires arch code to initialize a list of
> struct pages, and it requires the mm core to walk the entire list
> just to figure out how long it is), and it prevents arch code from
> doing anything fancy when a special mapping fault occurs.
> 
> Add a .fault method as an alternative to filling in a .pages array.
> 
> ...
>
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -568,10 +568,27 @@ static inline void clear_tlb_flush_pending(struct mm_struct *mm)
>  }
>  #endif
>  
> +struct vm_fault;
> +
>  struct vm_special_mapping
>  {

We may as well fix the code layout while we're in there.

> -	const char *name;
> +	const char *name;	/* The name, e.g. "[vdso]". */
> +
> +	/*
> +	 * If .fault is not provided, this is points to a

s/is//

> +	 * NULL-terminated array of pages that back the special mapping.
> +	 *
> +	 * This must not be NULL unless .fault is provided.
> +	 */
>  	struct page **pages;
> +
> +	/*
> +	 * If non-NULL, then this is called to resolve page faults
> +	 * on the special mapping.  If used, .pages is not checked.
> +	 */
> +	int (*fault)(const struct vm_special_mapping *sm,
> +		     struct vm_area_struct *vma,
> +		     struct vm_fault *vmf);
>  };
>  
>  enum tlb_flush_reason {
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 2ce04a649f6b..f717453b1a57 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3030,11 +3030,16 @@ static int special_mapping_fault(struct vm_area_struct *vma,
>  	pgoff_t pgoff;
>  	struct page **pages;
>  
> -	if (vma->vm_ops == &legacy_special_mapping_vmops)
> +	if (vma->vm_ops == &legacy_special_mapping_vmops) {
>  		pages = vma->vm_private_data;
> -	else
> -		pages = ((struct vm_special_mapping *)vma->vm_private_data)->
> -			pages;
> +	} else {
> +		struct vm_special_mapping *sm = vma->vm_private_data;
> +
> +		if (sm->fault)
> +			return sm->fault(sm, vma, vmf);
> +
> +		pages = sm->pages;
> +	}
>  
>  	for (pgoff = vmf->pgoff; pgoff && *pages; ++pages)
>  		pgoff--;

Otherwise looks OK.  I'll assume this will be merged via an x86 tree.

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