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 ... ". ***
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?
Ewart
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