On Thu, Oct 30, 2014 at 9:38 PM, zhanghailiang <zhang.zhanghailiang@xxxxxxxxxx> 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 >> >> -- >> To unsubscribe from this list: send the line "unsubscribe kvm" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> . >> > -- Andres Lagar-Cavilla | Google Kernel Team | andreslc@xxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
