linux-kernel - Re: [PATCH 5/8] mm: vmscan: replace shrink

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Message-ID: <20191025134443.GA385668@cmpxchg.org>
Date:   Fri, 25 Oct 2019 09:44:43 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 5/8] mm: vmscan: replace shrink_node() loop with a retry
 jump

On Wed, Oct 23, 2019 at 04:18:57PM +0200, Michal Hocko wrote:
> On Tue 22-10-19 10:48:00, Johannes Weiner wrote:
> > Most of the function body is inside a loop, which imposes an
> > additional indentation and scoping level that makes the code a bit
> > hard to follow and modify.
> 
> I do agree!
> 
> > The looping only happens in case of reclaim-compaction, which isn't
> > the common case. So rather than adding yet another function level to
> > the reclaim path and have every reclaim invocation go through a level
> > that only exists for one specific cornercase, use a retry goto.
> 
> I would just keep the core logic in its own function and do the loop
> around it rather than a goto retry. This is certainly a matter of taste
> but I like a loop with an explicit condition much more than a if with
> goto.

Yeah, as the changelog says, I'm intentionally putting the looping
construct into the "cold path" of the code flow: we only loops in a
very specific cornercase, and having the whole body in a loop, or
creating another function nesting level for it suggests otherwise.

A goto seems like the perfect tool to have a retry for one particular
caller without muddying the code flow for the common call stack.

Matter of taste, I guess.