Re: [PATCH v3 06/13] mm: add remap_pfn_range_prepare(), remap_pfn_range_complete()

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Sep 16, 2025 at 03:11:52PM +0100, Lorenzo Stoakes wrote:
> We need the ability to split PFN remap between updating the VMA and
> performing the actual remap, in order to do away with the legacy
> f_op->mmap hook.
> 
> To do so, update the PFN remap code to provide shared logic, and also make
> remap_pfn_range_notrack() static, as its one user, io_mapping_map_user()
> was removed in commit 9a4f90e24661 ("mm: remove mm/io-mapping.c").
> 
> Then, introduce remap_pfn_range_prepare(), which accepts VMA descriptor
> and PFN parameters, and remap_pfn_range_complete() which accepts the same
> parameters as remap_pfn_rangte().
> 
> remap_pfn_range_prepare() will set the cow vma->vm_pgoff if necessary, so
> it must be supplied with a correct PFN to do so.  If the caller must hold
> locks to be able to do this, those locks should be held across the
> operation, and mmap_abort() should be provided to revoke the lock should
> an error arise.

It looks like the patches have changed to remove mmap_abort so this
paragraph can probably be dropped.

>  static int remap_pfn_range_internal(struct vm_area_struct *vma, unsigned long addr,
> -		unsigned long pfn, unsigned long size, pgprot_t prot)
> +		unsigned long pfn, unsigned long size, pgprot_t prot, bool set_vma)
>  {
>  	pgd_t *pgd;
>  	unsigned long next;
> @@ -2912,32 +2931,17 @@ static int remap_pfn_range_internal(struct vm_area_struct *vma, unsigned long ad
>  	if (WARN_ON_ONCE(!PAGE_ALIGNED(addr)))
>  		return -EINVAL;
>  
> -	/*
> -	 * Physically remapped pages are special. Tell the
> -	 * rest of the world about it:
> -	 *   VM_IO tells people not to look at these pages
> -	 *	(accesses can have side effects).
> -	 *   VM_PFNMAP tells the core MM that the base pages are just
> -	 *	raw PFN mappings, and do not have a "struct page" associated
> -	 *	with them.
> -	 *   VM_DONTEXPAND
> -	 *      Disable vma merging and expanding with mremap().
> -	 *   VM_DONTDUMP
> -	 *      Omit vma from core dump, even when VM_IO turned off.
> -	 *
> -	 * There's a horrible special case to handle copy-on-write
> -	 * behaviour that some programs depend on. We mark the "original"
> -	 * un-COW'ed pages by matching them up with "vma->vm_pgoff".
> -	 * See vm_normal_page() for details.
> -	 */
> -	if (is_cow_mapping(vma->vm_flags)) {
> -		if (addr != vma->vm_start || end != vma->vm_end)
> -			return -EINVAL;
> -		vma->vm_pgoff = pfn;
> +	if (set_vma) {
> +		err = get_remap_pgoff(vma->vm_flags, addr, end,
> +				      vma->vm_start, vma->vm_end,
> +				      pfn, &vma->vm_pgoff);
> +		if (err)
> +			return err;
> +		vm_flags_set(vma, VM_REMAP_FLAGS);
> +	} else {
> +		VM_WARN_ON_ONCE((vma->vm_flags & VM_REMAP_FLAGS) != VM_REMAP_FLAGS);
>  	}

It looks like you can avoid the changes to add set_vma by making
remap_pfn_range_internal() only do the complete portion and giving
the legacy calls a helper to do prepare in line:

int remap_pfn_range_prepare_vma(..)
{
	int err;

	err = get_remap_pgoff(vma->vm_flags, addr, end,
			      vma->vm_start, vma->vm_end,
			      pfn, &vma->vm_pgoff);
	if (err)
		return err;
	vm_flags_set(vma, VM_REMAP_FLAGS);
	return 0;
}

int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
	    	    unsigned long pfn, unsigned long size, pgprot_t prot)
{
	int err;

	err = remap_pfn_range_prepare_vma(vma, addr, pfn, size)
	if (err)
	     return err;

	if (IS_ENABLED(__HAVE_PFNMAP_TRACKING))
		return remap_pfn_range_track(vma, addr, pfn, size, prot);
	return remap_pfn_range_notrack(vma, addr, pfn, size, prot);
}

(fix pgtable_Types.h to #define to 1 so IS_ENABLED works)

But the logic here is all fine

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason