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

[zhanghailiang <zhang.zhanghailiang@huawei.com>]

On 2014/11/20 2:49, Andrea Arcangeli wrote:
> Hi Zhang,
>
> On Fri, Oct 31, 2014 at 09:26:09AM +0800, zhanghailiang wrote:
>> On 2014/10/30 20:49, Dr. David Alan Gilbert wrote:
>>> * zhanghailiang (zhang.zhanghailiang@huawei.com) wrote:
>>>> On 2014/10/30 1:46, Andrea Arcangeli wrote:
>>>>> Hi Zhanghailiang,
>>>>>
>>>>> On Mon, Oct 27, 2014 at 05:32:51PM +0800, zhanghailiang wrote:
>>>>>> Hi Andrea,
>>>>>>
>>>>>> Thanks for your hard work on userfault;)
>>>>>>
>>>>>> This is really a useful API.
>>>>>>
>>>>>> 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.
>>>>>>
>>>>>> I think this will help supporting vhost-scsi,ivshmem for migration,
>>>>>> we can trace dirty page in userspace.
>>>>>>
>>>>>> Actually, i'm trying to relize live memory snapshot based on pre-copy and userfault,
>>>>>> but reading memory from migration thread will also trigger userfault.
>>>>>> It will be easy to implement live memory snapshot, if we support configuring
>>>>>> userfault for writing memory only.
>>>>>
>>>>> 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.
>>>>>
>>>>> After some chat during the KVMForum I've been already thinking it
>>>>> could be beneficial for some usage to give userland the information
>>>>> about the fault being read or write, combined with the ability of
>>>>> mapping pages wrprotected to mcopy_atomic (that would work without
>>>>> false positives only with MADV_DONTFORK also set, but it's already set
>>>>> in qemu). That will require "vma->vm_flags & VM_USERFAULT" to be
>>>>> checked also in the wrprotect faults, not just in the not present
>>>>> faults, but it's not a massive change. Returning the read/write
>>>>> information is also a not massive change. This will then payoff mostly
>>>>> if there's also a way to remove the memory atomically (kind of
>>>>> remap_anon_pages).
>>>>>
>>>>> Would that be enough? I mean are you still ok if non present read
>>>>> fault traps too (you'd be notified it's a read) and you get
>>>>> notification for both wrprotect and non present faults?
>>>>>
>>>> Hi Andrea,
>>>>
>>>> Thanks for your reply, and your patience;)
>>>>
>>>> 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*.
>>>>
>>>> My initial solution scheme for live memory snapshot is:
>>>> (1) pause VM
>>>> (2) using userfaultfd to mark all memory of VM is wrprotect (readonly)
>>>> (3) save deivce state to snapshot file
>>>> (4) resume VM
>>>> (5) snapshot thread begin to save page of memory to snapshot file
>>>> (6) VM is going to run, and it is OK for VM or other thread to read ram (no fault trap),
>>>>       but if VM try to write page (dirty the page), there will be
>>>>       a userfault trap notification.
>>>> (7) a fault-handle-thread reads the page request from userfaultfd,
>>>>       it will copy content of the page to some buffers, and then remove the page's
>>>>       wrprotect limit(still using the userfaultfd to tell kernel).
>>>> (8) after step (7), VM can continue to write the page which is now can be write.
>>>> (9) snapshot thread save the page cached in step (7)
>>>> (10) repeat step (5)~(9) until all VM's memory is saved to snapshot file.
>>>
>>> Hmm, I can see the same process being useful for the fault-tolerance schemes
>>> like COLO, it needs a memory state snapshot.
>>>
>>>> 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.
>>>
>>> What pages would be non-present at this point - just balloon?
>>>
>>
>> Er, sorry, it should be 'no-present page faults';)
>
> Could you elaborate? The balloon pages or not yet allocated pages in
> the guest, if they fault too (in addition to the wrprotect faults) it
> doesn't sound a big deal, as it's not so common (balloon especially
> shouldn't happen except during balloon deflating during the live

> snapshotting). We could bypass non-present faults though, and only
> track strict wrprotect faults.
>

Yes, you are right. This is what i really want, bypass all non-present faults
and only track strict wrprotect faults. ;)

So, do you plan to support that in the userfault API?

Thanks,
zhanghailiang