From: Mel Gorman <mgorman@techsingularity.net>
To: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, Vlastimil Babka <vbabka@suse.cz>,
Michal Hocko <mhocko@suse.com>,
Nathan Chancellor <natechancellor@gmail.com>,
Nick Desaulniers <ndesaulniers@google.com>,
linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com,
kernel-janitors@vger.kernel.org, linux-safety@lists.elisa.tech
Subject: Re: [PATCH] mm/vmscan: drop unneeded assignment in kswapd()
Date: Sun, 4 Oct 2020 20:24:37 +0100 [thread overview]
Message-ID: <20201004192437.GF3227@techsingularity.net> (raw)
In-Reply-To: <20201004125827.17679-1-lukas.bulwahn@gmail.com>
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@gmail.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
next prev parent reply other threads:[~2020-10-04 19:24 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-04 12:58 [PATCH] mm/vmscan: drop unneeded assignment in kswapd() Lukas Bulwahn
2020-10-04 19:24 ` Mel Gorman [this message]
2020-10-05 6:58 ` Lukas Bulwahn
2020-10-05 7:56 ` Mel Gorman
2020-10-05 7:59 ` Mel Gorman
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