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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 <pjp@fedoraproject.org> Reviewed-by: Fabiano Rosas <farosas@suse.de> Link: https://lore.kernel.org/r/20250512125124.147064-2-ppandit@redhat.com Signed-off-by: Peter Xu <peterx@redhat.com>
112 lines
3 KiB
C
112 lines
3 KiB
C
/*
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* Multifd zero page detection implementation.
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*
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* Copyright (c) 2024 Bytedance Inc
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*
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* Authors:
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* Hao Xiang <hao.xiang@bytedance.com>
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*
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* This work is licensed under the terms of the GNU GPL, version 2 or later.
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* See the COPYING file in the top-level directory.
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*/
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#include "qemu/osdep.h"
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#include "qemu/cutils.h"
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#include "system/ramblock.h"
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#include "migration.h"
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#include "migration-stats.h"
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#include "multifd.h"
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#include "options.h"
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#include "ram.h"
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static bool multifd_zero_page_enabled(void)
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{
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return migrate_zero_page_detection() == ZERO_PAGE_DETECTION_MULTIFD;
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}
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static void swap_page_offset(ram_addr_t *pages_offset, int a, int b)
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{
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ram_addr_t temp;
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if (a == b) {
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return;
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}
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temp = pages_offset[a];
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pages_offset[a] = pages_offset[b];
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pages_offset[b] = temp;
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}
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/**
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* multifd_send_zero_page_detect: Perform zero page detection on all pages.
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*
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* Sorts normal pages before zero pages in p->pages->offset and updates
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* p->pages->normal_num.
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*
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* @param p A pointer to the send params.
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*/
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void multifd_send_zero_page_detect(MultiFDSendParams *p)
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{
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MultiFDPages_t *pages = &p->data->u.ram;
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RAMBlock *rb = pages->block;
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int i = 0;
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int j = pages->num - 1;
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if (!multifd_zero_page_enabled()) {
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pages->normal_num = pages->num;
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goto out;
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}
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/*
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* Sort the page offset array by moving all normal pages to
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* the left and all zero pages to the right of the array.
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*/
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while (i <= j) {
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uint64_t offset = pages->offset[i];
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if (!buffer_is_zero(rb->host + offset, multifd_ram_page_size())) {
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i++;
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continue;
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}
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swap_page_offset(pages->offset, i, j);
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ram_release_page(rb->idstr, offset);
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j--;
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}
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pages->normal_num = i;
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out:
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stat64_add(&mig_stats.normal_pages, pages->normal_num);
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stat64_add(&mig_stats.zero_pages, pages->num - pages->normal_num);
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}
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void multifd_recv_zero_page_process(MultiFDRecvParams *p)
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{
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for (int i = 0; i < p->zero_num; i++) {
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void *page = p->host + p->zero[i];
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bool received =
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ramblock_recv_bitmap_test_byte_offset(p->block, p->zero[i]);
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/*
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* During multifd migration zero page is written to the memory
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* only if it is migrated more than once.
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*
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* It becomes a problem when both multifd & postcopy options are
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* enabled. If the zero page which was skipped during multifd phase,
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* is accessed during the postcopy phase of the migration, a page
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* fault occurs. But this page fault is not served because the
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* 'receivedmap' says the zero page is already received. Thus the
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* thread accessing that page may hang.
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*
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* When postcopy is enabled, always write the zero page as and when
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* it is migrated.
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*/
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if (migrate_postcopy_ram() || received) {
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memset(page, 0, multifd_ram_page_size());
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}
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if (!received) {
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ramblock_recv_bitmap_set_offset(p->block, p->zero[i]);
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}
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}
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}
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