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

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Date: Wed, 12 Jun 2024 11:42:47 +0100
From: Usama Arif <usamaarif642@...il.com>
To: Nhat Pham <nphamcs@...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 11/06/2024 20:33, Nhat Pham wrote:
> 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?

Yes, should add in swap_free_cluster as well, will do in next revision. 
Thanks!