Re: kernel panic due to https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2830bf6f05fb3e05bc4743274b806c821807a684

[Michal Hocko]

On Fri 25-01-19 18:33:15, Michal Hocko wrote:
> On Fri 25-01-19 17:39:38, Michal Hocko wrote:
> > On Fri 25-01-19 11:16:30, robert shteynfeld wrote:
> > > Attached is the dmesg from patched kernel.
> > 
> > Your Node1 physical memory range precedes Node0 which is quite unusual
> > but it shouldn't be a huge problem on its own. But memory ranges are
> > not aligned to the memory section
> > 
> > [    0.286954] Early memory node ranges
> > [    0.286955]   node   1: [mem 0x0000000000001000-0x0000000000090fff]
> > [    0.286955]   node   1: [mem 0x0000000000100000-0x00000000dbdf8fff]
> > [    0.286956]   node   1: [mem 0x0000000100000000-0x0000001423ffffff]
> > [    0.286956]   node   0: [mem 0x0000001424000000-0x0000002023ffffff]
> > 
> > As you can see the last pfn for the node1 is inside the section and
> > Node0 starts right after. This is quite unusual as well. If for no other
> > reasons then the memmap of those struct pages will be remote for one or
> > the other. Actually I am not even sure we can handle that properly
> > because we do expect 1:1 mapping between sections and nodes.
> > 
> > Now it also makes some sense why 2830bf6f05fb ("mm, memory_hotplug:
> > initialize struct pages for the full memory section") made any
> > difference. We simply write over a potentially initialized struct page
> > and blow up on that. I strongly suspect that the commit just uncovered
> > a pre-existing problem. Let me think what we can do about that.
> 
> Appart from force aligning node's start the only other option is to
> revert 2830bf6f05fb and handling the underlying issue in the hotplug
> code.

We cannot really align because we have things like ZONE_DMA starting at
0x1000 and who knows what else. So let's go with the revert. Hutplug
simply needs a larger surgery to get rid of the PAGES_PER_SECTION
inherent assumptions.

Linus, could you take the revert please?