Re: [LSF/MM/BPF TOPIC] State Of The Page

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On Wed, Mar 1, 2023 at 11:03 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Mar 01, 2023 at 10:50:24PM -0500, Pasha Tatashin wrote:
> > On Tue, Feb 21, 2023 at 2:58 PM Matthew Wilcox <willy@infradead.org> wrote:
> > > My goal for 2023 is to get to a point where we (a) have struct page
> > > reduced to:
> > >
> > > struct page {
> > >         unsigned long flags;
> > >         struct list_head lru;
> > >         struct address_space *mapping;
> > >         pgoff_t index;
> > >         unsigned long private;
> > >         atomic_t _mapcount;
> > >         atomic_t _refcount;
> > >         unsigned long memcg_data;
> > > #ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS
> > >         int _last_cpupid;
> > > #endif
> > > };
> >
> > This looks clean, but it is still 64-bytes. I wonder if we could
> > potentially reduce it down to 56 bytes by removing memcg_data.
>
> We need struct page to be 16-byte aligned to make slab work.  We also need
> it to divide PAGE_SIZE evenly to make CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP

Hm, can you please elaborate on both of these cases, how do both of
these cases work today with _last_cpuid configs or some other configs
that increase "struct page" above 64-bytes?

> work.  I don't think it's worth nibbling around the edges like this
> anyway; convert everything from page to folio and then we can do the
> big bang conversion where struct page shrinks from 64 bytes to 8.

I agree with general idea that converting to folio and shrinking
"struct page" to 8 bytes can be a big memory consumption win, but even
then we do not want to encourage the memdesc users to use larger than
needed types. If "flags" and "memcgs" are going to be part of almost
every single memdesc type it would be nice to reduce them from
16-bytes to 8-bytes.

Pasha