On Tue, Jun 11, 2024 at 11:50 AM Usama Arif <usamaarif642@xxxxxxxxx> wrote:
> In swap_writepage, with this patch you have:
>
> if (is_folio_zero_filled(folio)) {
> swap_zeromap_folio_set(folio);
> folio_unlock(folio);
> return 0;
> }
> swap_zeromap_folio_clear(folio);
>
I was concerned with the swap slot being freed and reused, without
ever being read :) But looks like it has to be properly reset before
being reused, so all is well on that front.
What about the put_swap_folio() -> swap_free_cluster() case - do we
need to handle zeromap bit clearing here too? Looks like it's clearing
the swap_map (i.e returning it directly to the swapfile, allowing
those slots to be reused) here, and I notice that you clear the
zeromap bitmap wherever the swap_map is cleared as well :)
I jumped around the code a bit - in free_cluster() (called by
swap_free_cluster()), there's this chunk:
if ((si->flags & (SWP_WRITEOK | SWP_PAGE_DISCARD)) ==
(SWP_WRITEOK | SWP_PAGE_DISCARD)) {
swap_cluster_schedule_discard(si, idx);
return;
}
swap_cluster_schedule_discard() does clear_bit() on the zeromap on the
entire cluster. We also clear_bit() in the work function
swap_do_scheduled_discard() (is this redundant?).
But what if this check is false, i.e the swap device does not have the
SWP_PAGE_DISCARD flag set? Are we not clearing the bits in the zeromap
here?
