Re: [mm 4.15-rc8] Random oopses under memory pressure.

[Andrea Arcangeli]

On Thu, Jan 18, 2018 at 06:45:00AM -0800, Dave Hansen wrote:
> On 01/18/2018 04:25 AM, Kirill A. Shutemov wrote:
> > [   10.084024] diff: -858690919
> > [   10.084258] hpage_nr_pages: 1
> > [   10.084386] check1: 0
> > [   10.084478] check2: 0
> ...
> > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> > index d22b84310f6d..57b4397f1ea5 100644
> > --- a/mm/page_vma_mapped.c
> > +++ b/mm/page_vma_mapped.c
> > @@ -70,6 +70,14 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw)
> >  		}
> >  		if (pte_page(*pvmw->pte) < pvmw->page)
> >  			return false;
> > +
> > +		if (pte_page(*pvmw->pte) - pvmw->page) {
> > +			printk("diff: %d\n", pte_page(*pvmw->pte) - pvmw->page);
> > +			printk("hpage_nr_pages: %d\n", hpage_nr_pages(pvmw->page));
> > +			printk("check1: %d\n", pte_page(*pvmw->pte) - pvmw->page < 0);
> > +			printk("check2: %d\n", pte_page(*pvmw->pte) - pvmw->page >= hpage_nr_pages(pvmw->page));
> > +			BUG();
> > +		}
> 
> This says that pte_page(*pvmw->pte) and pvmw->page are roughly 4GB away
> from each other (858690919*4=0xccba559c0).  That's not the compiler
> being wonky, it just means that the virtual addresses of the memory
> sections are that far apart.
> 
> This won't happen when you have vmemmap or flatmem because the mem_map[]
> is virtually contiguous and pointer arithmetic just works against all
> 'struct page' pointers.  But with classic sparsemem, it doesn't.
> 
> You need to make sure that the PFNs are in the same section before you
> can do the math that you want to do here.

Isn't it simply that pvmw->page isn't a page or pte_page(*pvmw->pte)
isn't a page?

The distance cannot matter, MMU isn't involved, this is pure 64bit
aritmetics, 1giga 1 terabyte, 48bits 5level pagetables are meaningless
in this comparison.

#include <stdio.h>

int main()
{
	volatile long i;
	struct x { char a[4000000000]; };
	for (i = 0; i < 4000000000*3; i += 4000000000) {
		printf("%ld\n", ((struct x *)0)-((((struct x *)i))));
	}
	printf("xxxx\n");
	for (i = 0; i < 4000000000; i += 1) {
		if (i==4)
			i = 4000000000;
		printf("%ld\n", ((struct x *)0)-((((struct x *)i))));
	}
	return 0;
}

You need to add two debug checks on "pte_page(*pvmw->pte) % 64" and
same for pvmw->page to find out the one of the two that isn't a page.

If both are real pages there's a bug that allocates page structs not
naturally aligned.