Re: [LSF/MM/BPF TOPIC] Single Owner Memory

[Matthew Wilcox <willy@infradead.org>]

On Mon, Feb 20, 2023 at 02:10:24PM -0500, Pasha Tatashin wrote:
> The discussion should include the following topics:
> -  Interaction with folio and the proposed struct page {memdesc}.
> - Handling for migrate_pages() and friends.
> - Handling for FOLL_PIN and FOLL_LONGTERM.
> - What type of madvise() properties the som memory should handle

Something I didn't see covered was how you'd want to handle memory
pressure.  The answer for memdescs is that we'd treat each userspace
allocation as a single object; if you allocate a 256kB folio, that has
one accessed bit (set every time any of the PTEs which reference that
folio is accessed), one dirty bit, is aged on the LRU as a single unit
and will be written to swap as a single unit.

Assuming we're dealing with objects smaller than PMDs, we have a number
of PTEs each of which has its own A and D bits, so we can determine
at each revolution of the LRU clock whether it still makes sense to be
treating the folio as a single unit, or whether pages in the first half
of the folio are no longer being accessed and we should split the folio
in half and age the two halves separately.

All of that is still theoretical; we don't allocate anon memory in sizes
other than PAGE_SIZE and PMD size.  And we don't track page cache A and
D bits to see whether the decision to allocate a particular page size was
the right one (most page cache memory is never mapped into userspace, so
it might be of limited value, but I'm sure we could track the equivalent
information with read() and write()).