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

["Alastair D'Silva" <alastair@d-silva.org>]

On Tue, 2019-07-02 at 08:13 +0200, Michal Hocko wrote:
> On Tue 02-07-19 14:13:25, Alastair D'Silva wrote:
> > 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.
> 
> Just make it clear, I am not against a sanitization. I am objecting
> to
> put it into __section_nr because this is way too late. As already
> explained, you already must have a bogus mem_section object in hand.
> Why cannot you add a sanity check right there when the memory is
> added?
> Either when the section is registered or even sooner in
> arch_add_memory.
> 

Good point, I was thinking of a libnvdimm enhancement to check that the
end address is in range, but a more generic solution is better.

-- 
Alastair D'Silva           mob: 0423 762 819
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