linux-kernel - Re: [PATCH v2 1/3] mm: Trigger bug on if a section is not found in __section

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Message-ID: <7f0ac9250e6fe6318aaf0685be56b121a978ce1b.camel@d-silva.org>
Date:   Tue, 02 Jul 2019 14:13:25 +1000
From:   "Alastair D'Silva" <alastair@...ilva.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Oscar Salvador <osalvador@...e.de>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Baoquan He <bhe@...hat.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 1/3] mm: Trigger bug on if a section is not found in
 __section_nr

On Mon, 2019-07-01 at 12:46 +0200, Michal Hocko wrote:
> On Fri 28-06-19 10:46:28, Alastair D'Silva wrote:
> [...]
> > Given that there is already a VM_BUG_ON in the code, how do you
> > feel
> > about broadening the scope from 'VM_BUG_ON(!root)' to
> > 'VM_BUG_ON(!root
> > > > (root_nr == NR_SECTION_ROOTS))'?
> 
> As far as I understand the existing VM_BUG_ON will hit when the
> mem_section tree gets corrupted. This is a different situation to an
> incorrect section given so I wouldn't really mix those two. And I
> still
> do not see much point to protect from unexpected input parameter as
> this
> is internal function as already pointed out.
> 

Hi Michael,

I was able to hit this problem as the system firmware had assigned the
prototype pmem device an address range above the 128TB limit that we
originally supported. This has since been lifted to 2PB with patch
4ffe713b7587b14695c9bec26a000fc88ef54895.

As it stands, we cannot move this range lower as the high bits are
dictated by the location the card is connected.

Since the physical address of the memory is not controlled by the
kernel, I believe we should catch (or at least make it easy to debug)
the sitution where external firmware allocates physical addresses
beyond that which the kernel supports.

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