Corrupted pmap pm_vlist - pmap_remove_pte()

[Konstantin Belousov]

On Mon, Apr 16, 2012 at 03:08:25PM -0400, Ewart Tempest wrote:
> In FreeBSD 6.*, we have been seeing crashes in pmap_remove_pages() that only seem to occur in scaling scenarios:
> 
> 2564    #ifdef PMAP_REMOVE_PAGES_CURPROC_ONLY
> 2565                    pte = vtopte(pv->pv_va);
> 2566    #else
> 2567                    pte = pmap_pte(pmap, pv->pv_va);
> 2568    #endif
> 2569                    tpte = *pte; <===================== page fault here
> 
> The suspicion is that the pmap's pm_pvlist list is getting corrupted. To this end, I have a question on the following logic in pmap_remove_pte() (see in-line comment):
> 
>    1533 static int
>    1534 pmap_remove_pte(pmap_t pmap, pt_entry_t *ptq, vm_offset_t va, pd_entry_t ptepde)
>    1535 {
>    1536 	pt_entry_t oldpte;
>    1537 	vm_page_t m;
>    1538 
>    1539 	PMAP_LOCK_ASSERT(pmap, MA_OWNED);
>    1540 	oldpte = pte_load_clear(ptq);
>    1541 	if (oldpte & PG_W)
>    1542 		pmap->pm_stats.wired_count -= 1;
>    1543 	/*
>    1544 	 * Machines that don't support invlpg, also don't support
>    1545 	 * PG_G.
>    1546 	 */
>    1547 	if (oldpte & PG_G)
>    1548 		pmap_invalidate_page(kernel_pmap, va);
>    1549 	pmap->pm_stats.resident_count -= 1;
>    1550 	if (oldpte & PG_MANAGED) {
>    1551 		m = PHYS_TO_VM_PAGE(oldpte & PG_FRAME);
>    1552 		if (oldpte & PG_M) {
>    1553 #if defined(PMAP_DIAGNOSTIC)
>    1554 			if (pmap_nw_modified((pt_entry_t) oldpte)) {
>    1555 				printf(
>    1556 	"pmap_remove: modified page not writable: va: 0x%lx, pte: 0x%lx\n",
>    1557 				    va, oldpte);
>    1558 			}
>    1559 #endif
>    1560 			if (pmap_track_modified(va))
>    1561 				vm_page_dirty(m);
>    1562 		}
>    1563 		if (oldpte & PG_A)
>    1564 			vm_page_flag_set(m, PG_REFERENCED);
>    1565 		pmap_remove_entry(pmap, m, va);
>    1566 	}
>    1567 	return (pmap_unuse_pt(pmap, va, ptepde)); <======= *** under what circumstances is it valid to free the page but not remove it from the pmap's pm_vlist? Even the code comment for pmap_unuse_pt() commences "After removing a page table entry ... ". ***

It is valid to not remove pv_entry when no pv_entry exists for the mapping.
The pv_entry is created if the page is managed, see pmap_enter() code.
The block above the return is executed when the page is managed, or at
least pmap thinks so.

The HEAD code will panic in pmap_pvh_free() if pmap_phv_remove() cannot
find the pv entry for given page and given pmap/va.

>    1568 }
> 
> If the tail end of the above function is changed as follows:
> 
>    1565 		pmap_remove_entry(pmap, m, va);
>    1565.5 	      return (pmap_unuse_pt(pmap, va, ptepde));
>    1566 	}
>    1567 	return (0);
> 
> Then we don't see any crashes ... but is it the right thing to do?
Should be not. Try to test this with some unmanaged mapping, like
/dev/mem pages mapped into the exiting process address space.

I am too new to know about any nuances of the RELENG_6 code.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 196 bytes
Desc: not available
Url : http://lists.freebsd.org/pipermail/freebsd-hackers/attachments/20120417/17a9c840/attachment.pgp