Re: kernel BUG in page_add_anon_rmap

[Liam R. Howlett]

* Hugh Dickins <hughd@xxxxxxxxxx> [230129 01:49]:
> On Fri, 27 Jan 2023, Hugh Dickins wrote:
> > On Fri, 27 Jan 2023, David Hildenbrand wrote:
> > > On 26.01.23 19:57, Matthew Wilcox wrote:
> > > > On Wed, Jan 25, 2023 at 11:59:16PM +0000, Sanan Hasanov wrote:
> > > >> Good day, dear maintainers,
> > > >>
> > > >> We found a bug using a modified kernel configuration file used by syzbot.
> > > >>
> > > >> We enhanced the coverage of the configuration file using our tool,
> > > >> klocalizer.
> > > >>
> > > >> Kernel Branch: 6.2.0-rc5-next-20230124
> > > >> Kernel
> > > >> config: https://drive.google.com/file/d/1MZSgIF4R9QfikEuF5siUIZVPce-GiJQK/view?usp=sharing
> > > >> Reproducer: https://drive.google.com/file/d/1H5KWkT9VVMWTUVVgIaZi6J-fmukRx-BM/view?usp=sharing
> > > >>
> > > >> Thank you!
> > > >>
> > > >> Best regards,
> > > >> Sanan Hasanov
> 
> This is a very interesting find: the thanks go to you.
> 

...

> Upstream's fine; on next-20230127 (with David's repro) it bisects to
> 5ddaec50023e ("mm/mmap: remove __vma_adjust()").  I think I'd better
> hand on to Liam, rather than delay you by puzzling over it further myself.

Thanks Hugh!

...

> > > Indeed, the mapcount of the subpage is 2 instead of 1. The subpage is only
> > > mapped into a single
> > > page table (no fork() or similar).
> 
> Yes, that mapcount:2 is weird; and what's also weird is the index:0x20003:
> what is remove_migration_pte(), in an mbind(0x20002000,...), doing with
> index:0x20003?
> 
> My guess is that the remove-__vma_adjust() commit is not properly updating
> vm_pgoff into non_vma in some case: so that when remove_migration_pte()
> looks for where to insert the new pte, it's off by one page.

That looks to be exactly correct.  I am setting the vm_pgoff to the
wrong value in case 8 (for lack of a better name).

> 
> > > 
> > > I created this reduced reproducer that triggers 100%:
> 
> Very helpful, thank you.

Yes, thank you very much for find this bug and the reproducer.

...

Thanks,
Liam