Messages in this thread | | | Date | Fri, 31 Oct 2014 12:38:48 +0800 | From | zhanghailiang <> | Subject | Re: [PATCH 00/17] RFC: userfault v2 |
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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.
>>> 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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