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

[Michal Hocko <mhocko@kernel.org>]

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.

-- 
Michal Hocko
SUSE Labs