linux-kernel - Re: [PATCH v4 2/2] mm: remove code to handle same filled pages

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Date: Wed, 12 Jun 2024 08:09:01 -0700
From: Nhat Pham <nphamcs@...il.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, shakeel.butt@...ux.dev, 
	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
Subject: Re: [PATCH v4 2/2] mm: remove code to handle same filled pages

On Wed, Jun 12, 2024 at 5:47 AM Usama Arif <usamaarif642@...il.com> wrote:
>
> With an earlier commit to handle zero-filled pages in swap directly,
> and with only 1% of the same-filled pages being non-zero, zswap no
> longer needs to handle same-filled pages and can just work on compressed
> pages.
>
> Signed-off-by: Usama Arif <usamaarif642@...il.com>
> ---
>  mm/zswap.c | 86 +++++-------------------------------------------------
>  1 file changed, 8 insertions(+), 78 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index b9b35ef86d9b..ca8df9c99abf 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -44,8 +44,6 @@
>  **********************************/
>  /* The number of compressed pages currently stored in zswap */
>  atomic_t zswap_stored_pages = ATOMIC_INIT(0);
> -/* The number of same-value filled pages currently stored in zswap */
> -static atomic_t zswap_same_filled_pages = ATOMIC_INIT(0);

Can we re-introduce this counter somewhere? I have found this counter
to be valuable in the past, and would love to keep it. For instance,
in a system where a lot of zero-filled pages are reclaimed, we might
see an increase in swap usage. Having this counter at hands will allow
us to trace the source of the swap usage more easily. I *suppose* you
can do elimination work (check zswap usage, check disk swap usage
somehow - I'm not sure, etc., etc.), but this would be much more
direct and user-friendly.

Not *entirely* sure where to expose this though. Seems a bit specific
for vmstat. Maybe as a new debugfs directory?

This doesn't have to be done in this patch (or this series even) - but
please consider this for follow-up work at least :)