Re: [PATCH] Xen: Fix retry calls into PRIVCMD_MMAPBATCH*.

[David Vrabel <david.vrabel@citrix.com>]

On 01/08/13 12:49, Andres Lagar-Cavilla wrote:
> On Aug 1, 2013, at 7:23 AM, David Vrabel <david.vrabel@citrix.com> wrote:
> 
>> On 01/08/13 04:30, Andres Lagar-Cavilla wrote:
>>> -- Resend as I haven't seen this hit the lists. Maybe some smtp misconfig. Apologies. Also expanded cc --
>>>
>>> When a foreign mapper attempts to map guest frames that are paged out,
>>> the mapper receives an ENOENT response and will have to try again
>>> while a helper process pages the target frame back in.
>>>
>>> Gating checks on PRIVCMD_MMAPBATCH* ioctl args were preventing retries
>>> of mapping calls.
>>
>> This breaks the auto_translated_physmap case as will allocate another
>> set of empty pages and leak the previous set.
> 
> David,
> not able to follow you here. Under what circumstances will another
> set of empty pages be allocated? And where? are we talking page table pages?

	....
	vma = find_vma(mm, m.addr);
	if (!vma ||
	    vma->vm_ops != &privcmd_vm_ops ||
	    (m.addr != vma->vm_start) ||
	    ((m.addr + (nr_pages << PAGE_SHIFT)) != vma->vm_end) ||
	    !privcmd_enforce_singleshot_mapping(vma)) {
		up_write(&mm->mmap_sem);
		ret = -EINVAL;
		goto out;
	}
	if (xen_feature(XENFEAT_auto_translated_physmap)) {
		ret = alloc_empty_pages(vma, m.num);

Here.

		if (ret < 0) {
			up_write(&mm->mmap_sem);
			goto out;
		}
	}


>> This privcmd_enforce_singleshot_mapping() stuff seems very odd anyway.
>> Does anyone know what it was for originally?  It would be preferrable if
>> we could update the mappings with a new set of foreign MFNs without
>> having to tear down the VMA and recreate a new VMA.
> 
> I believe it's mostly historical. I agree with you on principle, but recreating VMAs is super-cheap.

Tearing them down is not cheap as each page requires a trap-and-emulate
to clear the PTE (see ptep_get_and_clear_full() in zap_pte_range()).

David