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Message-ID: <e66e43b1fdfbff94ab23a23c48aa6cbe210a3131.camel@d-silva.org>
Subject: Re: [PATCH v2 1/3] mm: Trigger bug on if a section is not found in
 __section_nr
From:   "Alastair D'Silva" <alastair@d-silva.org>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Pavel Tatashin <pasha.tatashin@oracle.com>,
        Oscar Salvador <osalvador@suse.de>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Baoquan He <bhe@redhat.com>,
        Wei Yang <richard.weiyang@gmail.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Date:   Thu, 27 Jun 2019 10:50:57 +1000
In-Reply-To: <20190626065751.GK17798@dhcp22.suse.cz>
References: <20190626061124.16013-1-alastair@au1.ibm.com>
         <20190626061124.16013-2-alastair@au1.ibm.com>
         <20190626062113.GF17798@dhcp22.suse.cz>
         <d4af66721ea53ce7df2d45a567d17a30575672b2.camel@d-silva.org>
         <20190626065751.GK17798@dhcp22.suse.cz>
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On Wed, 2019-06-26 at 08:57 +0200, Michal Hocko wrote:
> On Wed 26-06-19 16:27:30, Alastair D'Silva wrote:
> > On Wed, 2019-06-26 at 08:21 +0200, Michal Hocko wrote:
> > > On Wed 26-06-19 16:11:21, Alastair D'Silva wrote:
> > > > From: Alastair D'Silva <alastair@d-silva.org>
> > > > 
> > > > If a memory section comes in where the physical address is
> > > > greater
> > > > than
> > > > that which is managed by the kernel, this function would not
> > > > trigger the
> > > > bug and instead return a bogus section number.
> > > > 
> > > > This patch tracks whether the section was actually found, and
> > > > triggers the
> > > > bug if not.
> > > 
> > > Why do we want/need that? In other words the changelog should
> > > contina
> > > WHY and WHAT. This one contains only the later one.
> > >  
> > 
> > Thanks, I'll update the comment.
> > 
> > During driver development, I tried adding peristent memory at a
> > memory
> > address that exceeded the maximum permissable address for the
> > platform.
> > 
> > This caused __section_nr to silently return bogus section numbers,
> > rather than complaining.
> 
> OK, I see, but is an additional code worth it for the non-development
> case? I mean why should we be testing for something that shouldn't
> happen normally? Is it too easy to get things wrong or what is the
> underlying reason to change it now?
> 

It took me a while to identify what the problem was - having the BUG_ON
would have saved me a few hours.

I'm happy to just have the BUG_ON 'nd drop the new error return (I
added that in response to Mike Rapoport's comment that the original
patch would still return a bogus section number).


-- 
Alastair D'Silva           mob: 0423 762 819
skype: alastair_dsilva    
Twitter: @EvilDeece
blog: http://alastair.d-silva.org