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

["David Hildenbrand (Arm)" <david@kernel.org>]

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.

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);
 }


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.

-- 
Cheers,

David