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

[Pasha Tatashin]

On Tue, Feb 21, 2023 at 2:58 PM Matthew Wilcox <willy at infradead.org> wrote:
>
> On Wed, Feb 22, 2023 at 02:08:28AM +0800, Gao Xiang wrote:
> > On 2023/1/27 00:40, Matthew Wilcox wrote:
> > > I'd like to do another session on how the struct page dismemberment
> > > is going and what remains to be done.  Given how widely struct page is
> > > used, I think there will be interest from more than just MM, so I'd
> > > suggest a plenary session.
> >
> > I'm interested in this topic too, also I'd like to get some idea of the
> > future of the page dismemberment timeline so that I can have time to keep
> > the pace with it since some embedded use cases like Android are
> > memory-sensitive all the time.
>
> As you all know, I'm absolutely amazing at project management & planning
> and can tell you to the day when a feature will be ready ;-)
>
> 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.
Something like this might work:

1. On a 64-bit system flags field contains 19 unused bits, we could
potentially use the free bits in this field.
2. There are up-to 64K memcg ids. So in case this field contains memcg
pointer 16-bit id would be enough to convert to memcg pointer
3. In case memcg_data contains a pointer to a list of memcgs, there
could be a separate hash table data structure that contains pointers
to memcgs for slabs, or other users.

However, I am not sure how that would affect the performance, but it
would be very nice to reduce "struct page" by  8-bytes.

Pasha