Re: [PATCH v2 2/2] mm/hugetlb: support write-faults in shared mappings

David Hildenbrand
2022-08-16T20:44:21Z

Hi Gerald,

>
> Thanks, we were also trying to reproduce on x86, w/o success so far. But
> I guess that matches David latest observations wrt to our exception handling
> code on s390.
>
> Good news is that the problem goes away when I add this simple patch, which
> should result in proper VM_WRITE check for vma flags, before triggering a
> FAULT_FLAG_WRITE fault:
>
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -379,7 +379,9 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
> flags = FAULT_FLAG_DEFAULT;
> if (user_mode(regs))
> flags |= FAULT_FLAG_USER;
> - if (access == VM_WRITE || is_write)
> + if (is_write)
> + access = VM_WRITE;
> + if (access == VM_WRITE)
> flags |= FAULT_FLAG_WRITE;
> mmap_read_lock(mm);

That's what I had in mind, good.

>
> Still find it a bit hard to believe that this > 10 years old logic really
> is/was broken all the time. I guess it simply did not matter for normal
> PTE faults, probably because the common fault handling code later would
> check itself via maybe_mkwrite(). And for hugetlb PTEs, it might not have
> mattered before commit bcd51a3c679d.

It is akward, but maybe we never really noticed for hugetlb (not sure
how common read-only mappings are after all).

>
> >
> > bcd51a3c679d eliminates the copying of page tables at fork for non-anon
> > hugetlb vmas. So, in these tests you would likely see more pte_none()
> > faults.
>
> Yes, makes sense, assuming now that it actually is related to s390
> exception handling code, not checking for VM_WRITE before triggering a
> write fault for pte_none().
>
> Thanks for checking! And Thanks a lot to David for finding that issue
> in s390 exception handling code!

Thanks! Looks like adding the WARN_ON_ONCE was the right decision.