Re: [PATCH 00/17] RFC: userfault v2
From: zhanghailiang
Date: Fri Oct 31 2014 - 04:13:04 EST
On 2014/10/31 13:17, Andres Lagar-Cavilla wrote:
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.
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
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