Re: [PATCH v4 0/6] UFFD write-tracking migration/snapshots

[Peter Xu]

On Thu, Nov 26, 2020 at 06:17:28PM +0300, Andrey Gruzdev wrote:
> Changes v3->v4:
> 
> * 1. Renamed migrate capability 'track-writes-ram'->'background-snapshot'.
> * 2. Use array of incompatible caps to replace bulky 'if' constructs.
> * 3. Moved UFFD low-level code to the separate module ('util/userfaultfd.c').
> * 4. Always do UFFD wr-unprotect on cleanup; just closing file descriptor
> *    won't cleanup PTEs anyhow, it will release registration ranges, wait 
> *    queues etc. but won't cleanup process MM context on MMU level.
> * 5. Allow to enable 'background-snapshot' capability on Linux-only hosts.
> * 6. Put UFFD code usage under '#ifdef CONFIG_LINUX' prerequisite.
> * 7. Removed 'wt_' from RAMState struct.
> * 8. Refactored ram_find_and_save_block() to make more clean - poll UFFD
> *    wr-fault events in get_queued_page(), use ram_save_host_page_pre(),
> *    ram_save_host_page_post() notifiers around ram_save_host_page()
> *    instead of bulky inline write-unprotect code.

One thing I mentioned previously but it seems still got lost is that we don't
need dirty tracking for live snapshot.

A few pointers for reference:

  memory_global_dirty_log_start()
  migration_bitmap_sync_precopy()
  memory_region_clear_dirty_bitmap()
  ...

These should not be needed.  But this can also be done on top.

Thanks,

-- 
Peter Xu