[..]
> >
> > /*
> > * 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;
> }
>
