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

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Message-ID: <20200320155505.GA204561@cmpxchg.org>
Date:   Fri, 20 Mar 2020 11:55:05 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     mateusznosek0@...il.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH] mm/vmscan.c: Clean code by removing unnecessary
 assignment

On Thu, Mar 19, 2020 at 06:13:34PM +0100, Michal Hocko wrote:
> 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.
> "

Yeah that sounds good.

> > 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.

It's a valid concern, but I think in this case specifically we're very
unlikely to change the ordering here - violate memory.low before going
after active pages of unprotected cgroups.

I indeed just kept it stupid: reset everything, then retry. But it
appears that the unnecessary assignment trips people up and wastes
their time, so I'm in favor of removing it.

Acked-by: Johannes Weiner <hannes@...xchg.org>