linux-kernel - Re: [PATCH v3 1/2] mm: store zero pages to be swapped out in a bitmap

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Date: Tue, 11 Jun 2024 12:33:29 -0700
From: Nhat Pham <nphamcs@...il.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, david@...hat.com, 
	ying.huang@...el.com, hughd@...gle.com, willy@...radead.org, 
	yosryahmed@...gle.com, chengming.zhou@...ux.dev, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, kernel-team@...a.com, 
	Shakeel Butt <shakeel.butt@...ux.dev>
Subject: Re: [PATCH v3 1/2] mm: store zero pages to be swapped out in a bitmap

On Tue, Jun 11, 2024 at 11:50 AM Usama Arif <usamaarif642@...il.com> 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?