Re: [PATCH 00/17] RFC: userfault v2

[Andres Lagar-Cavilla <andreslc@google.com>]

On Thu, Oct 30, 2014 at 9:38 PM, zhanghailiang
<zhang.zhanghailiang@huawei.com> wrote:
> On 2014/10/31 11:29, zhanghailiang wrote:
>>
>> On 2014/10/31 10:23, Peter Feiner wrote:
>>>
>>> On Thu, Oct 30, 2014 at 07:31:48PM +0800, zhanghailiang wrote:
>>>>
>>>> On 2014/10/30 1:46, Andrea Arcangeli wrote:
>>>>>
>>>>> On Mon, Oct 27, 2014 at 05:32:51PM +0800, zhanghailiang wrote:
>>>>>>
>>>>>> I want to confirm a question:
>>>>>> Can we support distinguishing between writing and reading memory for
>>>>>> userfault?
>>>>>> That is, we can decide whether writing a page, reading a page or both
>>>>>> trigger userfault.
>>>>>
>>>>> Mail is going to be long enough already so I'll just assume tracking
>>>>> dirty memory in userland (instead of doing it in kernel) is worthy
>>>>> feature to have here.
>>>
>>>
>>> I'll open that can of worms :-)
>>>
>>>> [...]
>>>> Er, maybe i didn't describe clearly. What i really need for live memory
>>>> snapshot
>>>> is only wrprotect fault, like kvm's dirty tracing mechanism, *only
>>>> tracing write action*.
>>>>
>>>> So, what i need for userfault is supporting only wrprotect fault. i
>>>> don't
>>>> want to get notification for non present reading faults, it will
>>>> influence
>>>> VM's performance and the efficiency of doing snapshot.
>>>
>>>
>>> Given that you do care about performance Zhanghailiang, I don't think
>>> that a
>>> userfault handler is a good place to track dirty memory. Every dirtying
>>> write
>>> will block on the userfault handler, which is an expensively slow
>>> proposition
>>> compared to an in-kernel approach.
>>>
>>
>> Agreed, but for doing live memory snapshot (VM is running when do
>> snapsphot),
>> we have to do this (block the write action), because we have to save the
>> page before it
>> is dirtied by writing action. This is the difference, compared to pre-copy
>> migration.
>>
>
> Again;) For snapshot, i don't use its dirty tracing ability, i just use it
> to block write action,
> and save page, and then i will remove its write protect.

You could do a CoW in the kernel, post a notification, keep going, and
expose an interface for user-space to mmap the preserved copy. Getting
the life-cycle of the preserved page(s) right is tricky, but doable.
Anyway, it's easy to hand-wave without knowing your specific
requirements.

Opening the discussion a bit, this does look similar to the xen-access
interface, in which a xen domain vcpu could be stopped in its tracks
while user-space was notified (and acknowledged) a variety of
scenarios: page was written to, page was read from, vcpu is attempting
to execute from page, etc. Very applicable to anti-viruses right away,
for example you can enforce W^X properties on pages.

I don't know that Andrea wants to open the game so broadly for
userfault, and the code right now is very specific to triggering on
pte_none(), but that's a nice reward down this road.

Andres

>
>>>> Also, i think this feature will benefit for migration of ivshmem and
>>>> vhost-scsi
>>>> which have no dirty-page-tracing now.
>>>
>>>
>>> I do agree wholeheartedly with you here. Manually tracking non-guest
>>> writes
>>> adds to the complexity of device emulation code. A central fault-driven
>>> means
>>> for dirty tracking writes from the guest and host would be a welcome
>>> simplification to implementing pre-copy migration. Indeed, that's exactly
>>> what
>>> I'm working on! I'm using the softdirty bit, which was introduced
>>> recently for
>>> CRIU migration, to replace the use of KVM's dirty logging and manual
>>> dirty
>>> tracking by the VMM during pre-copy migration. See
>>
>>
>> Great! Do you plan to issue your patches to community? I mean is your work
>> based on
>> qemu? or an independent tool (CRIU migration?) for live-migration?
>> Maybe i could fix the migration problem for ivshmem in qemu now,
>> based on softdirty mechanism.
>>
>>> Documentation/vm/soft-dirty.txt and pagemap.txt in case you aren't
>>> familiar. To
>>
>>
>> I have read them cursorily, it is useful for pre-copy indeed. But it seems
>> that
>> it can not meet my need for snapshot.
>>
>>> make softdirty usable for live migration, I've added an API to atomically
>>> test-and-clear the bit and write protect the page.
>>
>>
>> How can i find the API? Is it been merged in kernel's master branch
>> already?
>>
>>
>> Thanks,
>> zhanghailiang
>>
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>



-- 
Andres Lagar-Cavilla | Google Kernel Team | andreslc@google.com