Re: [PATCH v2 00/15] mm/rmap: index MAP_PRIVATE file-backed folios by virt pgoff

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Mon, Jul 27, 2026 at 04:44:06PM +0200, David Hildenbrand (Arm) wrote:
> On 7/20/26 16:38, Lorenzo Stoakes (ARM) wrote:
> > In memory management we've managed to manufacture a great deal of confusion
> > around the concept of anonymous memory. We have:
> >
> > 1. 'Pure anon' memory - anonymous VMAs whose folios are anonymous and
> >    swap-backed (thus for reclaim purposes, treated as anonymous). These are
> >    simple enough.
> >
> > 2. shmem - file-backed VMAs, file-backed folios (from rmap perspective) so
> >    present in the page cache and mapped by an address_space object, but
> >    whose folios are also swap-backed (thus treated as anonymous for reclaim
> >    purposes).
> >
> > 3. MAP_PRIVATE-mapped /dev/zero - a strange beast whose VMAs have
> >    vma->vm_file set, but whose mmap_prepare callback clears vma->vm_ops to
> >    satisfy vma_is_anonymous(), which results in VMAs that were mmap()'d
> >    referencing a file, but are in every other sense anonymous, including the
> >    folios.
> >
> > 4. Other MAP_PRIVATE-file backed mappings - These possess file-backed VMAs
> >    and have file-backed folios until CoW'd, at which point those CoW'd
> >    folios are anonymous.
> >
> > This series fixes issues 3 and 4.
> >
> > In order for us to traverse VMAs using the reverse mapping, we require two
> > fields - folio->mapping and folio->index. The first tells the rmap code
> > where to look for VMAs, and the second tells it at which offset the folio
> > starts within the referenced object.
> >
> > For anonymous folios, folio->mapping points at an anon_vma object. For
> > file-backed folios, it points at an address_space. And:
> >
> > * For file-backed folios folio->index is simply the page offset of the start
> >   of the folio within the file.
> >
> > * For anonymous folios belonging to pure anon mappings, folio->index is
> >   equal to the virtual page offset of the folio.
> >
> > * For anonymous folios belonging to file-backed mappings (i.e. CoW'd folios
> >   of a MAP_PRIVATE file-backed mapping), folio->index is equal to the file
> >   page offset.
> >
> > This series establishes a new virtual page offset property of VMAs to
> > allow us to map anonymous folios at their virtual page offset, consistent
> > with pure anon.
>
> As raised off-list, I consider the "virtual page offset" concept hard to grasp.
>
> Maybe it's just me :)
>
> Skimming over the code, I read "vmg->anon_pgoff", which is pretty intuitive to
> me. Similarly vma_start_anon_pgoff() / vmg_end_anon_pgoff().
>
> Could we similarly just call this "anon_pgoff" /  "(linear) anon page index"
> even on the VMA level.

Yeah, I had originally done this.

I renamed it because I worried that people might be confused by references to
anon for file-backed VMA mappings.

But I think probably you're right that referring to it as virtual page offset
adds even more confusion vs. simply calling it anon. Will respin accordingly.


>
> IOW, in patch #9 for example:
>
> static inline pgoff_t linear_folio_page_index(const struct folio *folio,
>  					      const struct vm_area_struct *vma,
>  					      const unsigned long address)
>  {
> 	if (folio_test_anon(folio))
> +		return linear_anon_page_index(vma, address);
> +
>  	return linear_page_index(vma, address);
>  }

Ack yeah.

>
>
> Or is there another user for the the "virtual page offset" concept? I'd assume
> it's only used for anon folios (including KSM), but maybe I am missing some
> corner case.

No it was just fear of adding more confusion but seems by doing so I've done the
reverse of what I wanted :P

When I rework it if I notice anything that promotes this as a better solution
will ping otherwise will just rename on respin.

>
> --
> Cheers,
>
> David

Cheers, Lorenzo