Messages in this thread | | | Date | Fri, 31 Oct 2014 16:11:24 +0800 | From | zhanghailiang <> | Subject | Re: [PATCH 00/17] RFC: userfault v2 |
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On 2014/10/31 13:17, Andres Lagar-Cavilla wrote: > 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. >
Yes, what i need is very much like user-space COW feature, but i don't want to modify any code of kvm to relize COW, usefault is a more generic way and more grace. Besides, I'm not an expert in kernel:(
> 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
Right;)
> 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. >
I hope he will consider it. IMHO, it is a good extension for userfault (write fault);)
Best Regards, zhanghailiang
>> >>>>> 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 >>> >>> -- >>> To unsubscribe from this list: send the line "unsubscribe kvm" in >>> the body of a message to majordomo@vger.kernel.org >>> More majordomo info at http://vger.kernel.org/majordomo-info.html >>> . >>> >> > > >
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