Re: [PATCH v1 0/3] migration: Fixes to the 'background-snapshot' code

[Andrey Gruzdev]

On 24.03.2021 01:21, Peter Xu wrote:

On Fri, Mar 19, 2021 at 05:52:46PM +0300, Andrey Gruzdev wrote:

Changes v0->v1:
  * Using qemu_real_host_page_size instead of TARGET_PAGE_SIZE for host
    page size in ram_block_populate_pages()
  * More elegant implementation of ram_block_populate_pages()

This patch series contains:
  * Fix to the issue with occasionally truncated non-iterable device state
  * Solution to compatibility issues with virtio-balloon device
  * Fix to the issue when discarded or never populated pages miss UFFD
    write protection and get into migration stream in dirty state

Andrey Gruzdev (3):
   migration: Fix missing qemu_fflush() on buffer file in
     bg_migration_thread
   migration: Inhibit virtio-balloon for the duration of background
     snapshot
   migration: Pre-fault memory before starting background snasphot

Unless Andrey would like to respin a new version, this version looks good to me
(I don't think the adding new helper issue in patch 1 is a blocker):

Reviewed-by: Peter Xu <pet...@redhat.com>

Thanks.

I'm also looking into introducing UFFD_FEATURE_WP_UNALLOCATED so as to
wr-protect page holes too for a uffd-wp region when the feature bit is set.
With that feature we should be able to avoid pre-fault as what we do in the
last patch of this series.  However even if that can work out, we'll still need
this for old kernel anyways.

I'm curious this new feature is based on adding wr-protection at the level of 
VMAs,
so we won't miss write faults for missing pages?

Thanks,