From: Prasad Pandit <p...@fedoraproject.org> During multifd migration, zero pages are written if they are migrated more than once.
This may result in a migration thread hang issue when multifd and postcopy are enabled together. When postcopy is enabled, always write zero pages as and when they are migrated. Signed-off-by: Prasad Pandit <p...@fedoraproject.org> --- migration/multifd-zero-page.c | 22 ++++++++++++++++++++-- 1 file changed, 20 insertions(+), 2 deletions(-) v11: - s/ones/once, remove capitalisations and call _test_byte_offset() once v10: - https://lore.kernel.org/qemu-devel/20250508122849.207213-2-ppan...@redhat.com/T/#t diff --git a/migration/multifd-zero-page.c b/migration/multifd-zero-page.c index dbc1184921..4cde868159 100644 --- a/migration/multifd-zero-page.c +++ b/migration/multifd-zero-page.c @@ -85,9 +85,27 @@ void multifd_recv_zero_page_process(MultiFDRecvParams *p) { for (int i = 0; i < p->zero_num; i++) { void *page = p->host + p->zero[i]; - if (ramblock_recv_bitmap_test_byte_offset(p->block, p->zero[i])) { + bool received = + ramblock_recv_bitmap_test_byte_offset(p->block, p->zero[i]); + + /* + * During multifd migration zero page is written to the memory + * only if it is migrated more than once. + * + * It becomes a problem when both multifd & postcopy options are + * enabled. If the zero page which was skipped during multifd phase, + * is accessed during the postcopy phase of the migration, a page + * fault occurs. But this page fault is not served because the + * 'receivedmap' says the zero page is already received. Thus the + * thread accessing that page may hang. + * + * When postcopy is enabled, always write the zero page as and when + * it is migrated. + */ + if (migrate_postcopy_ram() || received) { memset(page, 0, multifd_ram_page_size()); - } else { + } + if (!received) { ramblock_recv_bitmap_set_offset(p->block, p->zero[i]); } } -- 2.49.0
