linux-kernel - Re: [PATCH] mm/vmscan: drop unneeded assignment in kswapd()

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Date:   Sun, 4 Oct 2020 20:24:37 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Lukas Bulwahn <lukas.bulwahn@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        kernel-janitors@...r.kernel.org, linux-safety@...ts.elisa.tech
Subject: Re: [PATCH] mm/vmscan: drop unneeded assignment in kswapd()

On Sun, Oct 04, 2020 at 02:58:27PM +0200, Lukas Bulwahn wrote:
> The refactoring to kswapd() in commit e716f2eb24de ("mm, vmscan: prevent
> kswapd sleeping prematurely due to mismatched classzone_idx") turned an
> assignment to reclaim_order into a dead store, as in all further paths,
> reclaim_order will be assigned again before it is used.
> 
> make clang-analyzer on x86_64 tinyconfig caught my attention with:
> 
>   mm/vmscan.c: warning: Although the value stored to 'reclaim_order' is
>   used in the enclosing expression, the value is never actually read from
>   'reclaim_order' [clang-analyzer-deadcode.DeadStores]
> 
> Compilers will detect this unneeded assignment and optimize this anyway.
> So, the resulting binary is identical before and after this change.
> 
> Simplify the code and remove unneeded assignment to make clang-analyzer
> happy.
> 
> No functional change. No change in binary code.
> 
> Signed-off-by: Lukas Bulwahn <lukas.bulwahn@...il.com>

I'm not really keen on this. With the patch, reclaim_order can be passed
uninitialised to kswapd_try_to_sleep. While a sufficiently smart
compiler might be able to optimise how reclaim_order is used, it's not
guaranteed either. Similarly, a change in kswapd_try_to_sleep and its
called functions could rely on reclaim_order being a valid value and
then introduce a subtle bug.

-- 
Mel Gorman
SUSE Labs