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Subject: Re: UBSAN: Undefined behaviour in mm/page_alloc.c
References: <CAEAjamseRRHu+TaTkd1TwpLNm8mtDGP=2K0WKLF0wH-3iLcW_w@mail.gmail.com>
 <20181109084353.GA5321@dhcp22.suse.cz>
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 <20181109095604.GC5321@dhcp22.suse.cz>
From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <a74e6a5d-d4c1-9006-60af-de52afafebb2@i-love.sakura.ne.jp>
Date: Fri, 9 Nov 2018 19:07:49 +0900
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To: Michal Hocko <mhocko@kernel.org>
Cc: Kyungtae Kim <kt0755@gmail.com>, akpm@linux-foundation.org, 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>

On 2018/11/09 18:56, Michal Hocko wrote:
> Does this following look better?

Yes.

>> Also, why not to add BUG_ON(gfp_mask & __GFP_NOFAIL); here?
> 
> Because we do not want to blow up the kernel just because of a stupid
> usage of the allocator. Can you think of an example where it would
> actually make any sense?
> 
> I would argue that such a theoretical abuse would blow up on an
> unchecked NULL ptr access. Isn't that enough?

We after all can't avoid blowing up the kernel even if we don't add BUG_ON().
Stopping with BUG_ON() is saner than NULL pointer dereference messages.