Hi Zhang, On Fri, Oct 31, 2014 at 09:26:09AM +0800, zhanghailiang wrote: > On 2014/10/30 20:49, Dr. David Alan Gilbert wrote: > > * zhanghailiang (zhang.zhanghailiang@xxxxxxxxxx) 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. > > > > Hmm, I can see the same process being useful for the fault-tolerance schemes > > like COLO, it needs a memory state snapshot. > > > >> 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. > > > > What pages would be non-present at this point - just balloon? > > > > Er, sorry, it should be 'no-present page faults';) Could you elaborate? The balloon pages or not yet allocated pages in the guest, if they fault too (in addition to the wrprotect faults) it doesn't sound a big deal, as it's not so common (balloon especially shouldn't happen except during balloon deflating during the live snapshotting). We could bypass non-present faults though, and only track strict wrprotect faults. -- 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
