Messages in this thread | | | Date | Thu, 20 Nov 2014 10:54:29 +0800 | From | zhanghailiang <> | Subject | Re: [PATCH 00/17] RFC: userfault v2 |
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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
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