| 1 | /* |
| 2 | * Multifd zero page detection implementation. |
| 3 | * |
| 4 | * Copyright (c) 2024 Bytedance Inc |
| 5 | * |
| 6 | * Authors: |
| 7 | * Hao Xiang <hao.xiang@bytedance.com> |
| 8 | * |
| 9 | * This work is licensed under the terms of the GNU GPL, version 2 or later. |
| 10 | * See the COPYING file in the top-level directory. |
| 11 | */ |
| 12 | |
| 13 | #include "qemu/osdep.h" |
| 14 | #include "qemu/cutils.h" |
| 15 | #include "system/ramblock.h" |
| 16 | #include "migration.h" |
| 17 | #include "migration-stats.h" |
| 18 | #include "multifd.h" |
| 19 | #include "options.h" |
| 20 | #include "ram.h" |
| 21 | |
| 22 | static bool multifd_zero_page_enabled(void) |
| 23 | { |
| 24 | return migrate_zero_page_detection() == ZERO_PAGE_DETECTION_MULTIFD; |
| 25 | } |
| 26 | |
| 27 | static void swap_page_offset(ram_addr_t *pages_offset, int a, int b) |
| 28 | { |
| 29 | ram_addr_t temp; |
| 30 | |
| 31 | if (a == b) { |
| 32 | return; |
| 33 | } |
| 34 | |
| 35 | temp = pages_offset[a]; |
| 36 | pages_offset[a] = pages_offset[b]; |
| 37 | pages_offset[b] = temp; |
| 38 | } |
| 39 | |
| 40 | /** |
| 41 | * multifd_send_zero_page_detect: Perform zero page detection on all pages. |
| 42 | * |
| 43 | * Sorts normal pages before zero pages in p->pages->offset and updates |
| 44 | * p->pages->normal_num. |
| 45 | * |
| 46 | * @param p A pointer to the send params. |
| 47 | */ |
| 48 | void multifd_send_zero_page_detect(MultiFDSendParams *p) |
| 49 | { |
| 50 | MultiFDPages_t *pages = &p->data->u.ram; |
| 51 | RAMBlock *rb = pages->block; |
| 52 | int i = 0; |
| 53 | int j = pages->num - 1; |
| 54 | |
| 55 | if (!multifd_zero_page_enabled()) { |
| 56 | pages->normal_num = pages->num; |
| 57 | goto out; |
| 58 | } |
| 59 | |
| 60 | /* |
| 61 | * Sort the page offset array by moving all normal pages to |
| 62 | * the left and all zero pages to the right of the array. |
| 63 | */ |
| 64 | while (i <= j) { |
| 65 | uint64_t offset = pages->offset[i]; |
| 66 | |
| 67 | if (!buffer_is_zero(rb->host + offset, multifd_ram_page_size())) { |
| 68 | i++; |
| 69 | continue; |
| 70 | } |
| 71 | |
| 72 | swap_page_offset(pages->offset, i, j); |
| 73 | ram_release_page(rb->idstr, offset); |
| 74 | j--; |
| 75 | } |
| 76 | |
| 77 | pages->normal_num = i; |
| 78 | |
| 79 | out: |
| 80 | qatomic_add(&mig_stats.normal_pages, pages->normal_num); |
| 81 | qatomic_add(&mig_stats.zero_pages, pages->num - pages->normal_num); |
| 82 | } |
| 83 | |
| 84 | void multifd_recv_zero_page_process(MultiFDRecvParams *p) |
| 85 | { |
| 86 | for (int i = 0; i < p->zero_num; i++) { |
| 87 | void *page = p->host + p->zero[i]; |
| 88 | bool received = |
| 89 | ramblock_recv_bitmap_test_byte_offset(p->block, p->zero[i]); |
| 90 | |
| 91 | /* |
| 92 | * During multifd migration zero page is written to the memory |
| 93 | * only if it is migrated more than once. |
| 94 | * |
| 95 | * It becomes a problem when both multifd & postcopy options are |
| 96 | * enabled. If the zero page which was skipped during multifd phase, |
| 97 | * is accessed during the postcopy phase of the migration, a page |
| 98 | * fault occurs. But this page fault is not served because the |
| 99 | * 'receivedmap' says the zero page is already received. Thus the |
| 100 | * thread accessing that page may hang. |
| 101 | * |
| 102 | * When postcopy is enabled, always write the zero page as and when |
| 103 | * it is migrated. |
| 104 | */ |
| 105 | if (migrate_postcopy_ram() || received) { |
| 106 | memset(page, 0, multifd_ram_page_size()); |
| 107 | } |
| 108 | if (!received) { |
| 109 | ramblock_recv_bitmap_set_offset(p->block, p->zero[i]); |
| 110 | } |
| 111 | } |
| 112 | } |