On Wed, Dec 11, 2024 at 03:49:12PM +0100, David Hildenbrand wrote: > On 11.12.24 15:25, Ryan Roberts wrote: > > On 11/12/2024 11:56, David Hildenbrand wrote: > > > Now CCing the correct Willy :) > > > > > > On 11.12.24 12:55, David Hildenbrand wrote: > > > > Hi, > > > > > > > > PageAnonExclusive (PAE) is working very reliable at this point. But > > > > especially in the context of THPs (large folios) we'd like to do better: > > > > > > > > (1) For PTE-mapped THP, we have to maintain it per page. We'd like to > > > > avoid per-page flags as good as possible (e.g., waste in "struct > > > > page", touching many cachelines). > > > > Presumably also important for the Glorious Future where struct page is just a > > pointer and struct folio (et al) is allocated dynamically? > > I think Willy mentioned that there might be ways to encode it in the 8-byte > for the "tail" pages. Yes. For anon memory, the page->memdesc has a 4-bit 'type' and the remaining 60 bits is a pointer to a struct folio (allocated from a slab with 16 byte alignment). The current list of types [1] has file folios as type 2 and anon folios as type 3. We could allocate a type to be 'anon exclusive', thus essentially giving us an anon-exclusive bit. [1] https://kernelnewbies.org/MatthewWilcox/Memdescs Don't get too excited about "we're almost out of types". The "managed" type has subtypes. We could also collapse "file" and "anon" into a single type and distinguish between them with a bit in the folio. Anyway, yes, we can do one per-page flag. Two per-page flags starts to get dicey.
