Re: [PATCH v4 1/2] mm: store zero pages to be swapped out in a bitmap

[Yosry Ahmed <yosryahmed@google.com>]

[..]
> >
> > /*
> >  * Return the number of contiguous zeromap entries started from entry;
> >  * If all entries have consistent zeromap, *consistent will be true;
> >  * otherwise, false;
> >  */
> > static inline unsigned int swap_zeromap_entries_count(swp_entry_t entry,
> >                 int nr, bool *consistent)
> > {
> >         struct swap_info_struct *sis = swp_swap_info(entry);
> >         unsigned long start = swp_offset(entry);
> >         unsigned long end = start + nr;
> >         unsigned long s_idx, c_idx;
> >
> >         s_idx = find_next_bit(sis->zeromap, end, start);
> >         if (s_idx == end) {
> >                 *consistent = true;
> >                 return 0;
> >         }
> >
> >         c_idx = find_next_zero_bit(sis->zeromap, end, start);
> >         if (c_idx == end) {
> >                 *consistent = true;
> >                 return nr;
> >         }
> >
> >         *consistent = false;
> >         if (s_idx == start)
> >                 return 0;
> >         return c_idx - s_idx;
> > }
> >
> > I can actually switch the places of the "consistent" and returned
> > number if that looks
> > better.
>
> I'd rather make it simpler by:
>
> /*
>  * Check if all entries have consistent zeromap status, return true if
>  * all entries are zeromap or non-zeromap, else return false;
>  */
> static inline bool swap_zeromap_entries_check(swp_entry_t entry, int nr)
> {
>         struct swap_info_struct *sis = swp_swap_info(entry);
>         unsigned long start = swp_offset(entry);
>         unsigned long end = start + *nr;
>
>         if (find_next_bit(sis->zeromap, end, start) == end)
>                 return true;
>         if (find_next_zero_bit(sis->zeromap, end, start) == end)
>                 return true;
>
>         return false;
> }

We can start with a simple version like this, and when the time comes
to implement the logic below we can decide if it's worth the
complexity to return an exact number/order rather than a boolean to
decide the swapin order. I think it will also depend on whether we can
do the same for other backends (e.g. swapcache, zswap, etc). We can
note that in the commit log or something.

>
> mm/page_io.c can combine this with reading the zeromap of first entry to
> decide if it will read folio from zeromap; mm/memory.c only needs the bool
> to fallback to the largest possible order.
>
> static inline unsigned long thp_swap_suitable_orders(...)
> {
>         int order, nr;
>
>         order = highest_order(orders);
>
>         while (orders) {
>                 nr = 1 << order;
>                 if ((addr >> PAGE_SHIFT) % nr == swp_offset % nr &&
>                     swap_zeromap_entries_check(entry, nr))
>                         break;
>                 order = next_order(&orders, order);
>         }
>
>         return orders;
> }
>