linux-kernel - Re: [PATCH v3 06/13] mm: add remap_pfn_range_prepare(), remap_pfn_range

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Message-ID: <19ec3d37-46c4-4921-933d-4d554a351ef2@lucifer.local>
Date: Tue, 16 Sep 2025 18:37:57 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, Matthew Wilcox <willy@...radead.org>,
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Subject: Re: [PATCH v3 06/13] mm: add remap_pfn_range_prepare(),
 remap_pfn_range_complete()

On Tue, Sep 16, 2025 at 02:07:23PM -0300, Jason Gunthorpe wrote:
> 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.

Ugh, thought I'd caught all of these, oops. Will fixup...

>
> >  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:

OK nice, yeah I would always prefer to avoid a boolean parameter if possible.

Will do something similar to below.

>
> 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@...dia.com>

Thanks!

>
> Jason

Cheers, Lorenzo