Re: [patch 3/3] mm: fault handler to replace nopage and populate

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

> Yep, I see. You just need to be careful about the PFNMAP logic, so
> the VM knows whether the pte is backed by a struct page or not.

I still need to properly get my head around that one. I can't easily
change the VMA during the "switch" but I can tweak the flags on the
first nopage after one... 

> And going the pageless route means that you must disallow MAP_PRIVATE
> PROT_WRITE mappings, I trust that isn't a problem for you?

Should not but I need to look more closely.

> I guess the helper looks something like the following...
> 
> --
> 
> Index: linux-2.6/include/linux/mm.h
> ===================================================================
> --- linux-2.6.orig/include/linux/mm.h
> +++ linux-2.6/include/linux/mm.h
> @@ -1105,6 +1105,7 @@ unsigned long vmalloc_to_pfn(void *addr)
>  int remap_pfn_range(struct vm_area_struct *, unsigned long addr,
>  			unsigned long pfn, unsigned long size, pgprot_t);
>  int vm_insert_page(struct vm_area_struct *, unsigned long addr, struct page *);
> +int vm_insert_pfn(struct vm_area_struct *, unsigned long addr, unsigned long pfn);
>  
>  struct page *follow_page(struct vm_area_struct *, unsigned long address,
>  			unsigned int foll_flags);
> Index: linux-2.6/mm/memory.c
> ===================================================================
> --- linux-2.6.orig/mm/memory.c
> +++ linux-2.6/mm/memory.c
> @@ -1267,6 +1267,44 @@ int vm_insert_page(struct vm_area_struct
>  }
>  EXPORT_SYMBOL(vm_insert_page);
>  
> +/**
> + * vm_insert_pfn - insert single pfn into user vma
> + * @vma: user vma to map to
> + * @addr: target user address of this page
> + * @pfn: source kernel pfn
> + *
> + * Similar to vm_inert_page, this allows drivers to insert individual pages
> + * they've allocated into a user vma. Same comments apply
> + */
> +int vm_insert_pfn(struct vm_area_struct *vma, unsigned long addr, unsigned long pfn)
> +{
> +	struct mm_struct *mm = vma->vm_mm;
> +	int retval;
> +	pte_t *pte;
> +	spinlock_t *ptl;
> +
> +	BUG_ON(is_cow_mapping(vma->vm_flags));
> +
> +	retval = -ENOMEM;
> +	pte = get_locked_pte(mm, addr, &ptl);
> +	if (!pte)
> +		goto out;
> +	retval = -EBUSY;
> +	if (!pte_none(*pte))
> +		goto out_unlock;
> +
> +	/* Ok, finally just insert the thing.. */
> +	set_pte_at(mm, addr, pte, pfn_pte(pfn, vma->vm_page_prot));
> +
> +	vma->vm_flags |= VM_PFNMAP;
> +	retval = 0;
> +out_unlock:
> +	pte_unmap_unlock(pte, ptl);
> +out:
> +	return retval;
> +}
> +EXPORT_SYMBOL(vm_insert_pfn);

It also needs update_mmu_cache() I suppose.

>  /*
>   * maps a range of physical memory into the requested pages. the old
>   * mappings are removed. any references to nonexistent pages results

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