Re: PROBLEM: repeated remap_file_pages on tmpfs triggers bug on process exit

[Kirill A. Shutemov]

Konstantin Khlebnikov wrote:
> On Tue, Jul 15, 2014 at 2:55 PM, Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > Konstantin Khlebnikov wrote:
> >> It seems boundng logic in do_fault_around is wrong:
> >>
> >> start_addr = max(address & fault_around_mask(), vma->vm_start);
> >> off = ((address - start_addr) >> PAGE_SHIFT) & (PTRS_PER_PTE - 1);
> >> pte -= off;
> >> pgoff -= off;
> >>
> >> Ok, off  <= 511, but it might be bigger than pte offset in pte table.
> >
> > I don't see how it possible: fault_around_mask() cannot be more than 0x1ff000
> > (x86-64, fault_around_bytes == 2M). It means start_addr will be aligned to 2M
> > boundary in this case which is start of the page table pte belong to.
> >
> > Do I miss something?
> 
> Nope, you're right. This fixes kernel crash but not the original problem.
> 
> Problem is caused by calling do_fault_around for _non-linear_ faiult.
> In this case pgoff is shifted and might become negative during calculation.
> I'll send another patch.

I've got to the same conclusion. My patch is below.