Re: [PATCH 1/5] freepgt: free_pgtables use vma list

Benjamin Herrenschmidt
2005-03-22T23:31:22Z

On Tue, 2005-03-22 at 16:37 +0000, Hugh Dickins wrote:
> On Tue, 22 Mar 2005, Andrew Morton wrote:
> >
> > With these six patches the ppc64 is hitting the BUG in exit_mmap():
> >
> > BUG_ON(mm->nr_ptes); /* This is just debugging */
> >
> > fairly early in boot.
>
> So ppc64 is in the same boat as sparc64 (yet ia64 okay so far).
>
> Sorry, I'm still clueless.
>
> I cannot see those arches doing pte_allocs outside their vmas,
> that of course could cause it. And nr_ptes is initialized to 0
> once by memset and again by assignment, so it should be starting
> out even zeroer than most fields.

We do funny things in arch/ppc64/mm/init.c in the ioremap_mm, where we
don't use VMAs but our own mecanism (yah, ugly, but that's some legacy
we have from the original port, though I do intend to change that at one
point).

> I should probably be paying more attention to the repellent
> notion that my code is broken.
>
> If you and David could try the lame patch below,
> it'll at least give us a slight clue of where to be looking -
> every mm exiting with nr_ptes 1 means something different from
> every mm exiting with nr_ptes -1 means something different from
> occasional mms exiting with nr_ptes something positive.
>
> I'm not sure whether the patch would ever get to show a more
> interesting proc name than "?".
>
> And does memory leak away into lost pagetables if you continue
> running, or does it actually carry on running fine, and the
> problem appear to be with the BUG_ON itself?
>
> Thanks,
> Hugh
>
> --- freepgt6/mm/mmap.c 2005-03-22 04:28:40.000000000 +0000
> +++ testing/mm/mmap.c 2005-03-22 15:45:00.000000000 +0000
> @@ -1896,6 +1896,7 @@ EXPORT_SYMBOL(do_brk);
> /* Release all mmaps. */
> void exit_mmap(struct mm_struct *mm)
> {
> + static unsigned long good_mms, bad_mms;
> struct mmu_gather *tlb;
> struct vm_area_struct *vma = mm->mmap;
> unsigned long nr_accounted = 0;
> @@ -1931,7 +1932,14 @@ void exit_mmap(struct mm_struct *mm)
> vma = next;
> }
>
> - BUG_ON(mm->nr_ptes); /* This is just debugging */
> + if (mm->nr_ptes && bad_mms < 250) {
> + printk(KERN_ERR "exit_mmap: %s nr_ptes %ld good_mms %lu\n",
> + current->mm == mm? current->comm: "?",
> + (long)mm->nr_ptes, good_mms);
> + good_mms = 0;
> + bad_mms++;
> + } else
> + good_mms++;
> }
>
> /* Insert vm structure into process list sorted by address
--
Benjamin Herrenschmidt <benh@kernel.crashing.org>