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

[zhanghailiang]

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
>
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