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SubjectRe: [PATCH 00/17] RFC: userfault v2
Hi Andrea,

Is there any new about this discussion? ;)

Will you plan to support 'only wrprotect fault' in the userfault API?


On 2014/10/30 19:31, zhanghailiang wrote:
> On 2014/10/30 1:46, Andrea Arcangeli wrote:
>> Hi Zhanghailiang,
>> On Mon, Oct 27, 2014 at 05:32:51PM +0800, zhanghailiang wrote:
>>> Hi Andrea,
>>> Thanks for your hard work on userfault;)
>>> This is really a useful API.
>>> 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.
>>> I think this will help supporting vhost-scsi,ivshmem for migration,
>>> we can trace dirty page in userspace.
>>> Actually, i'm trying to relize live memory snapshot based on pre-copy and userfault,
>>> but reading memory from migration thread will also trigger userfault.
>>> It will be easy to implement live memory snapshot, if we support configuring
>>> userfault for writing memory only.
>> 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.
>> After some chat during the KVMForum I've been already thinking it
>> could be beneficial for some usage to give userland the information
>> about the fault being read or write, combined with the ability of
>> mapping pages wrprotected to mcopy_atomic (that would work without
>> false positives only with MADV_DONTFORK also set, but it's already set
>> in qemu). That will require "vma->vm_flags & VM_USERFAULT" to be
>> checked also in the wrprotect faults, not just in the not present
>> faults, but it's not a massive change. Returning the read/write
>> information is also a not massive change. This will then payoff mostly
>> if there's also a way to remove the memory atomically (kind of
>> remap_anon_pages).
>> Would that be enough? I mean are you still ok if non present read
>> fault traps too (you'd be notified it's a read) and you get
>> notification for both wrprotect and non present faults?
> Hi Andrea,
> Thanks for your reply, and your patience;)
> 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*.
> My initial solution scheme for live memory snapshot is:
> (1) pause VM
> (2) using userfaultfd to mark all memory of VM is wrprotect (readonly)
> (3) save deivce state to snapshot file
> (4) resume VM
> (5) snapshot thread begin to save page of memory to snapshot file
> (6) VM is going to run, and it is OK for VM or other thread to read ram (no fault trap),
> but if VM try to write page (dirty the page), there will be
> a userfault trap notification.
> (7) a fault-handle-thread reads the page request from userfaultfd,
> it will copy content of the page to some buffers, and then remove the page's
> wrprotect limit(still using the userfaultfd to tell kernel).
> (8) after step (7), VM can continue to write the page which is now can be write.
> (9) snapshot thread save the page cached in step (7)
> (10) repeat step (5)~(9) until all VM's memory is saved to snapshot file.
> 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.
> Also, i think this feature will benefit for migration of ivshmem and vhost-scsi
> which have no dirty-page-tracing now.
>> The question then is how you mark the memory readonly to let the
>> wrprotect faults trap if the memory already existed and you didn't map
>> it yourself in the guest with mcopy_atomic with a readonly flag.
>> My current plan would be:
>> fast path check in the not-present and wrprotect page fault
>> - if VM_USERFAULT is set, find if there's a userfaultfd registered
>> into that vma too
>> if yes engage userfaultfd protocol
>> otherwise raise SIGBUS (single threaded apps should be fine with
>> SIGBUS and it'll avoid them to spawn a thread in order to talk the
>> userfaultfd protocol)
>> - if userfaultfd protocol is engaged, return read|write fault + fault
>> address to read(ufd) syscalls
>> - leave the "userfault" resolution mechanism independent of the
>> userfaultfd protocol so we keep the two problems separated and we
>> don't mix them in the same API which makes it even harder to
>> finalize it.
>> add mcopy_atomic (with a flag to map the page readonly too)
>> The alternative would be to hide mcopy_atomic (and even
>> remap_anon_pages in order to "remove" the memory atomically for
>> the externalization into the cloud) as userfaultfd commands to
>> write into the fd. But then there would be no much point to keep
>> MADV_USERFAULT around if I do so and I could just remove it
>> too or it doesn't look clean having to open the userfaultfd just
>> to issue an hidden mcopy_atomic.
>> So it becomes a decision if the basic SIGBUS mode for single
>> threaded apps should be supported or not. As long as we support
>> SIGBUS too and we don't force to use userfaultfd as the only
>> mechanism to be notified about userfaults, having a separate
>> mcopy_atomic syscall sounds cleaner.
>> Perhaps mcopy_atomic could be used in other cases that may arise
>> later that may not be connected with the userfault.
>> Questions to double check the above plan is ok:
>> 1) should I drop the SIGBUS behavior and MADV_USERFAULT?
>> 2) should I hide mcopy_atomic as a write into the userfaultfd?
>> NOTE: even if I hide mcopy_atomic as a userfaultfd command to write
>> into the fd, the buffer pointer passed to write() syscall would
>> still _not_ be pointing to the data like a regular write, but it
>> would be a pointer to a command structure that points to the source
>> and destination data of the "hidden" mcopy_atomic, the only
>> advantage is that perhaps I could wakeup the blocked page faults
>> without requiring an additional syscall.
>> The standalone mcopy_atomic would still require a write into the
>> userfaultfd as it happens now after remap_anon_pages returns, in
>> order to wakeup the stopped page faults.
>> 3) should I add a registration command to trap only write faults?
> Sure, that is what i really need;)
> Best Regards,
> zhanghailiang
>> The protocol can always be extended later anyway in a backwards
>> compatible way but it's better if we get it fully featured from the
>> start.
>> For completeness, some answers for other questions I've seen floating
>> around but that weren't posted on the list yet (you can skip reading
>> the below part if not interested):
>> - open("/dev/userfault") instead of sys_userfaultfd(), I don't see the
>> benefit: userfaultfd is just like eventfd in terms of kernel API and
>> registering a /dev/ device actually sounds trickier. userfault is a
>> core VM feature and generally we prefer syscalls for core VM
>> features instead of running ioctl on some chardev that may or may
>> not exist. (like we did with /dev/ksm -> MADV_MERGEABLE)
>> - there was a suggestion during KVMForum about allowing an external
>> program to attach to any MM. Like ptrace. So you could have a single
>> process managing all userfaults for different processes. However
>> because I cannot allow multiple userfaultfd to register into the
>> same range, this doesn't look very reliable (ptrace is kind of an
>> optional/debug feature while if userfault goes wrong and returns
>> -EBUSY things go bad) and there may be other complications. If I'd
>> allow multiple userfaultfd to register into the same range, I
>> wouldn't even know who to deliver the userfault to. It is an erratic
>> behavior. Currently it'd return -EBUSY if the app has a bug and does
>> that, but maybe later this can be relaxed to allow higher
>> scalability with a flag (userfaultfd gets flags as parameters), but
>> it still would need to be the same logic that manages userfaults and
>> the only point of allowing multiple ufd to map the same range would
>> be SMP scalability. So I tend to see the userfaultfd as a MM local
>> thing. The thread managing the userfaults can still talk with
>> another process in the local machine using pipes or sockets if it
>> needs to.
>> - the userfaultfd protocol version handshake was done this way because
>> it looked more reliable.
>> Of course we could pass the version of the protocol as parameter to
>> userfaultfd too, but running the syscall multiple times until
>> -EPROTO didn't return anymore doesn't seem any better than writing
>> into the fd the wanted protocol until you read it back instead of
>> -1ULL. It just looked more reliable not having to run the syscall
>> again and again while depending on -EPROTO or some other
>> -Esomething.
>> Thanks,
>> Andrea
>> .
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