Re: [Xen-devel] xm pause causing lockup

[Kip Macy <kip.macy@xxxxxxxxx>]

> Does this happen if you boot with 'nosmp'? I don't really believe it's a
> race, but might be worth checking.

Yes, it still happens. It would have found it quite astonishing if it
were a race.
(XEN) EIP:    0808:[<fc52d5a3>]
(gdb) x/i 0xfc52d5a3
0xfc52d5a3 <get_page_type+265>: mov    0x14(%eax),%eax
(gdb) info line *0xfc52d5a3
Line 1236 of "mm.c" starts at address 0xfc52d5a0 <get_page_type+262>
and ends at 0xfc52d5b0 <get_page_type+278>.
(gdb) 

Line 1236-1240 of local mm.c:
            while ( (y = page->u.inuse.type_info) == x )
                cpu_relax();
            counter++;
            printk("page was not validated");
            goto again;

> Also, it's worth adding a printk into this loop just to check that that
> is where you're getting caught.

Obviously wasn't thinking and stuck it in the wrong place.
Nonetheless, even without the printk I think I've proven my point.


> 
>             /* Someone else is updating validation of this page. Wait...
> */
>             while ( (y = page->u.inuse.type_info) == x )
>                 cpu_relax();
>             goto again;

Yep.

> 
> We need to figure out how the type count managed to get to one without
> the page being validated. I presume you're doing a debug=y build of Xen?

Correct. Nothing comes out on the console apart from debug output from FreeBSD.

> Do you get any warnings about illegal mmu_update attempts when you boot
> FreeBSD?

No, I don't. This is the offending code snippet from pmap_pinit:

        /* install self-referential address mapping entry(s) */
        for (i = 0; i < NPGPTD; i++) {
                ma = xpmap_ptom(VM_PAGE_TO_PHYS(ptdpg[i]));
                pmap->pm_pdir[PTDPTDI + i] = ma | PG_V | PG_A | PG_M;
#ifdef PAE
                pmap->pm_pdpt[i] = ma | PG_V;
#endif
                /* re-map page directory read-only */
                PT_SET_MA(pmap->pm_pdir, *vtopte((vm_offset_t)pmap->pm_pdir) & 
~PG_RW);
                xen_pgd_pin(ma);
        }

PT_SET_MA is just a wrapper for update_va_mapping. Have there been any
recent changes to the page typing code that would cause it to get
confused by a self-referential mapping?

                          -Kip

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