On Mon, Jul 27, 2026 at 04:57:01PM +0100, Lorenzo Stoakes (ARM) wrote: > 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.
Would the term "logical page offset" be helpful? I already use this in my description of folios: * A folio is a physically, virtually and logically contiguous set * of bytes. It is a power-of-two in size, and it is aligned to that * same power-of-two. It is at least as large as %PAGE_SIZE. If it is * in the page cache, it is at a file offset which is a multiple of that * power-of-two. It may be mapped into userspace at an address which is * at an arbitrary page offset, but its kernel virtual address is aligned * to its size.
