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From: Michal Hocko <mhocko@kernel.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Kyungtae Kim <kt0755@gmail.com>,
	pavel.tatashin@microsoft.com, vbabka@suse.cz, osalvador@suse.de,
	rppt@linux.vnet.ibm.com, aaron.lu@intel.com,
	iamjoonsoo.kim@lge.com, alexander.h.duyck@linux.intel.com,
	mgorman@techsingularity.net, lifeasageek@gmail.com,
	threeearcat@gmail.com, syzkaller@googlegroups.com,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Subject: Re: UBSAN: Undefined behaviour in mm/page_alloc.c
Date: Wed, 14 Nov 2018 08:10:52 +0100
Message-ID: <20181114071052.GA23419@dhcp22.suse.cz> (raw)
In-Reply-To: <20181113152941.cc328e48d5c0c2f366f5db83@linux-foundation.org>

On Tue 13-11-18 15:29:41, Andrew Morton wrote:
[...]
> But do we really need to do this?  Are there any other known potential
> callsites?

The main point is that the code as it stands is quite fragile, isn't it?
Fixing up all the callers is possible but can you actually think of a
reason why this would cause any measurable effect in the fast path?
The order argument is usually in a register and comparing it to a number
with unlikely branch should be hardly something visible.

Besides that we are talking few cycles at best compared to a fragile
code that got broken by accident without anybody noticing for quite some
time.

I vote for the maintainability over few cycles here. Should anybody find
this measurable we can rework the code by other means.

-- 
Michal Hocko
SUSE Labs

  reply index

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-09  4:09 Kyungtae Kim
2018-11-09  8:42 ` Vlastimil Babka
2018-11-09  8:43 ` Michal Hocko
2018-11-09  9:41   ` Tetsuo Handa
2018-11-09  9:56     ` Michal Hocko
2018-11-09 10:07       ` Tetsuo Handa
2018-11-09 10:25         ` Michal Hocko
2018-11-09 10:10       ` Vlastimil Babka
2018-11-09 10:22         ` Michal Hocko
2018-11-09 10:24         ` Tetsuo Handa
2018-11-09 10:28           ` Michal Hocko
2018-11-09 10:52       ` Balbir Singh
2018-11-09  9:56   ` Mel Gorman
2018-11-13  9:43   ` Michal Hocko
2018-11-13 23:15     ` Andrew Morton
2018-11-13 23:23       ` Vlastimil Babka
2018-11-13 23:32         ` Andrew Morton
2018-11-14  0:05           ` Vlastimil Babka
2018-11-19 23:57             ` Pavel Machek
2018-11-13 23:29     ` Andrew Morton
2018-11-14  7:10       ` Michal Hocko [this message]
2018-11-16 23:43       ` Dmitry Vyukov

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