linux-kernel - Re: [PATCH] mm/vmscan.c: Clean code by removing unnecessary assignment

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Date:   Thu, 19 Mar 2020 18:13:34 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     mateusznosek0@...il.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH] mm/vmscan.c: Clean code by removing unnecessary
 assignment

It is usually preferable to Cc author of the code (added Johannes)

On Thu 19-03-20 17:59:38, mateusznosek0@...il.com wrote:
> From: Mateusz Nosek <mateusznosek0@...il.com>
> 
> Previously 0 was assigned to 'sc->skipped_deactivate'. It could happen only
> if 'sc->skipped_deactivate' was 0 so the assignment is unnecessary and can
> be removed.

The above wording was a bit hard to understdand for me. I would go with
"
sc->memcg_low_skipped resets skipped_deactivate to 0 but this is not
needed as this code path is never reachable with skipped_deactivate != 0
due to previous sc->skipped_deactivate branch.
"

> Signed-off-by: Mateusz Nosek <mateusznosek0@...il.com>

The patch is correct. I am not sure it results in a better code though.
I will defer to Johannes here. I suspect he simply wanted to express
that skipped_deactivate should be always reset when retrying the direct
reclaim. After this patch this could be lost in future changes so the
code would be more subtle. But I am only guessing here.

> ---
>  mm/vmscan.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index dca623db51c8..453ff2abcb58 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3093,7 +3093,6 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
>  	if (sc->memcg_low_skipped) {
>  		sc->priority = initial_priority;
>  		sc->force_deactivate = 0;
> -		sc->skipped_deactivate = 0;
>  		sc->memcg_low_reclaim = 1;
>  		sc->memcg_low_skipped = 0;
>  		goto retry;
> -- 
> 2.17.1

-- 
Michal Hocko
SUSE Labs