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.
