Re: [PATCH v4 08/11] migration: add eager load thread and setup for fast snapshot load

[Juraj Marcin <jmarcin@redhat.com>]

On 2026-08-01 08:06, Aadeshveer Singh wrote:
> In fast snapshot load a thread is needed for actively loading in pages
> along with the fault path so that the guest is not dependent on fault
> thread indefinitely. Considering the difference from usual network
> postcopy where major chunk of RAM is already loaded here entire RAM
> needs to be loaded later. Existance of background pages which are not
> really accessed by the guest might never be loaded and system will be
> locked in migration for indefinite time. As there should be no
> assumption about how guest accesses memory, the load times can be
> indefinite.
> 
> Add postcopy_ram_eager_load_thread(), for the eager thread which
> iterates over all non ignored blocks calling ram_block_load_eager()
> on each. ram_block_load_eager then iterates to load in all pages using
> postcopy_mapped_ram_load_page(), with a different channel, which takes
> care of not loading in pages already loaded by fault thread. On
> completion the thread schedules postcopy_incoming_complete_bh() to
> destroy the incoming migration state.
> 
> Add postcopy_ram_eager_load_setup() to create the thread. Added joining
> logic in postcopy_incoming_cleanup().
> 
> Add tracepoints for entry and exit to eager load thread.
> 
> When both mapped-ram and postcopy-ram are set, divert from
> qemu_loadvm_state to run fast snapshot load
> 
> Initialize postcopy RAM state and register RAM Blocks with userfaultfd
> via ram_postcopy_incoming_init() and postcopy_ram_incoming_setup() in
> process_incoming_migration_co(). Fault thread needs to be launched
> before VM to serve faults for some hardwares emulation that need to read
> RAM (like vapic devices). Populate bitmaps and offset tables while
> reading file in qemu_loadvm_state_main.
> 
> Add function qemu_loadvm_run_fast_snapshot_load() which starts the VM
> using loadvm_postcopy_handle_run_bh() and launches eager load thread.
> 
> Skip scheduling process_incoming_migration_bh() in
> process_incoming_migration_co(), for fast snapshot load as the state
> cleanup is managed by eager load thread on completion.
> 
> Signed-off-by: Aadeshveer Singh <aadeshveer07@gmail.com>
> Reviewed-by: Peter Xu <peterx@redhat.com>
> ---
>  migration/migration.c    | 36 +++++++++++++++++++--
>  migration/migration.h    |  5 +++
>  migration/postcopy-ram.c | 69 ++++++++++++++++++++++++++++++++++++++++
>  migration/postcopy-ram.h |  2 ++
>  migration/savevm.c       | 16 ++++++++++
>  migration/savevm.h       |  2 ++
>  migration/trace-events   |  2 ++
>  7 files changed, 130 insertions(+), 2 deletions(-)

Reviewed-by: Juraj Marcin <jmarcin@redhat.com>