Fix lazy mode vmalloc synchronization for paravirt

Found this looping Ubuntu installs with VMI.

If unlucky enough to hit a vmalloc sync fault during a lazy mode operation (from an IRQ handler for a module which was not yet populated in current page directory, or from inside copy_one_pte, which touches swap_map, and hit in an unused 4M region), the required PDE update would never get flushed, causing an infinite page fault loop.

This bug affects any paravirt-ops backend which uses lazy updates, I believe that makes it a bug in Xen, VMI and lguest. It only happens on LOWMEM kernels.

Currently for 2.6.23, but we'll want to backport to -stable as well.

Zach

Touching vmalloc memory in the middle of a lazy mode update can generate a kernel PDE update, which must be flushed immediately. The fix is to leave lazy mode when doing a vmalloc sync.

index 01ffdd4..fcb38e7 100644
@@ -249,9 +249,10 @@ static inline pmd_t *vmalloc_sync_one(pgd_t *pgd, unsigned long address)
 	pmd_k = pmd_offset(pud_k, address);
 	if (!pmd_present(*pmd_k))
 		return NULL;
-	if (!pmd_present(*pmd))
+	if (!pmd_present(*pmd)) {
 		set_pmd(pmd, *pmd_k);
-	else
+		arch_flush_lazy_mmu_mode();
+	} else
 		BUG_ON(pmd_page(*pmd) != pmd_page(*pmd_k));
 	return pmd_k;
 }

Ah, good catch.

No, under Xen the kernel/hypervisor PMD is not shared between processes, so this is still used when PAE is enabled.

J

Ahh, yes. So this was a lucky catch for us. Non-PAE kernels seem to be increasing in value at antique sales.

Zach