Re: [PATCH v3 00/11] migration: fast snapshot load

[Peter Xu <peterx@redhat.com>]

On Tue, Jul 14, 2026 at 07:45:36PM +0530, Aadeshveer Singh wrote:
> This series implements a "fast snapshot load" mechanism to
> significantly reduce the perceived resume time of a VM from a snapshot
> file.
> 
> Currently, resuming a VM from a snapshot file requires loading all RAM
> pages into the QEMU instance before execution begins. This extension
> allows the user to run the VM nearly instantly by loading only the
> required device states up front and loading RAM pages lazily, by
> trapping access to pages that have not yet been loaded.
> 
> Using the Linux userfaultfd syscall, a fault thread catches all page
> faults caused by the guest and loads in the pages required to keep
> the VM running. Concurrently, an eager background thread iteratively
> loads all remaining pages into RAM so the guest does not have to
> depend on the fault thread indefinitely.
> 
> Much of code is reused from postcopy for fault handling and precopy
> for reading mapped ram file. Implementation revolves around two
> threads named the fault thread and eager load thread. Fault thread as
> name suggests catches page faults by the guest and serves them using
> userfaultfd. Postcopy fault thread is reused but instead of requesting
> source for a page it loads the page directly by reading from file. In
> order to remove the dependency of guest on fault thread indefinitely
> the eager load thread loads in the entire RAM sequentially, and after
> iterating through the entire RAM signals fault thread to exit and
> calls cleanup.
> 
> In order to prevent the case of a page being loaded twice(in the
> case when eager load thread is loading it and fault thread also
> tries to serve fault on same page) a bitmap called pending_bmap is
> used to track pages which are pending and not being loaded by any
> thread. Atomic operations on this bitmap allows coordination between
> threads to prevent any unwanted behaviours
> 
> Future direction:
> - Add support for multifd
> - Add support for vhost-user

Please always still at least have a section for testing.  It's an important
piece of information on what you have tested (and what you have not, which
can also be included when it may matter).

For example, I know you should have tested at least normal and some huge
page setup, you can list them there.  You can also mention if you only
tested x86_64, or if you have played with anything else; logically this
should also work elsewhere whenever all facilities are available.  It's
fine to only test on x86_64, though.

Thanks,

> 
> ---
> v1 -> v2
> 
> - Structure Change: Expanded from 7 to 11 patches
> - Move test removal patch from first to post implementation as
>   suggested by Peter
> - Patch 1 to 5 are meant to be non functional changes meant to
>   modernize parts of code and prepare for feature Implementation
> - Patch 10 performs a smoke test for implemented feature
> - Patch 11 adds a documentation page for fast snapshot load
> - Added explicit fails for features not supported/incompatible with
>   fast snapshot load in compatibility check in options.c as suggested
>   by Peter
> - Passed errp object to qemu_get_buffer_at() to improve on error
>   reporting as suggested by Peter
> - Size of pending_bmap is updated to have a bit for every guest page
>   according to guest page size and not host page size as pointed
>   by Peter
> - New function(ramblock_file_bitmap_page_is_nonzero()) has been
>   introduced to test for zero pages while loading essentially enableing
>   support for hugepages
> - Updated the documentation comment over postcopy_mapped_ram_load_page()
>   to add errp as pointed by Peter
> - Added a new function try_mark_postcopy_blocktime_begin() to call
>   mark_postcopy_blocktime_begin() taking care of assumptions like
>   skipping if page is already received to improve on the readability
>   as pointed by Peter
> - Fixed bug, that sent non page alined addresses to
>   mark_postcopy_blocktime_begin() as pointed by Peter
> - Separate the postcopy_listen_thread_bh() rename patch as suggested
>   by Peter
> - Add comment explaining changes in process_incoming_migration_co()
>   as suggested by Peter
> - Make qemu_loadvm_run_fast_snapshot_load() and
>   postcopy_ram_eager_load_setup() return void as these functions
>   never fail as pointed by Peter
> 
> Aadeshveer Singh (11):
>   migration: Propagate error in postcopy setup functions
>   migration: Extract blocktime marking helper
>   migration: Rename postcopy_listen_thread_bh
>   migration: Use file_bmap for RAMBlock during incoming file load
>   migration: Make qemu_get_buffer_at() thread-safe
>   migration: add RAMBlock field and helper for fast snapshot load
>   migration: add support for fault thread to load pages from disk
>   migration: add eager load thread and setup for fast snapshot load
>   migration/tests: remove capability conflict test
>     postcopy-ram+mapped-ram
>   migration/tests: Add test for fast snapshot load
>   docs/migration: Add documentation for fast snapshot load feature
> 
>  docs/devel/migration/fast-snapshot-load.rst |  81 ++++++
>  docs/devel/migration/features.rst           |   1 +
>  include/system/ramblock.h                   |   6 +
>  migration/migration.c                       |  60 +++--
>  migration/migration.h                       |   5 +
>  migration/options.c                         |  14 +-
>  migration/postcopy-ram.c                    | 266 ++++++++++++++++----
>  migration/postcopy-ram.h                    |   7 +-
>  migration/qemu-file.c                       |   9 +-
>  migration/qemu-file.h                       |   4 +-
>  migration/ram.c                             | 103 +++++++-
>  migration/ram.h                             |   2 +
>  migration/savevm.c                          |  16 ++
>  migration/savevm.h                          |   2 +
>  migration/trace-events                      |   2 +
>  tests/qtest/migration/file-tests.c          |  24 ++
>  tests/qtest/migration/misc-tests.c          |  52 ----
>  17 files changed, 511 insertions(+), 143 deletions(-)
>  create mode 100644 docs/devel/migration/fast-snapshot-load.rst
> 
> -- 
> 2.55.0
> 

-- 
Peter Xu