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/*
* Multifd zero page detection implementation.
*
* Copyright (c) 2024 Bytedance Inc
*
* Authors:
* Hao Xiang <hao.xiang@bytedance.com>
*
* This work is licensed under the terms of the GNU GPL, version 2 or later.
* See the COPYING file in the top-level directory.
*/
#include "qemu/osdep.h"
#include "qemu/cutils.h"
#include "system/ramblock.h"
#include "migration.h"
#include "migration-stats.h"
#include "multifd.h"
#include "options.h"
#include "ram.h"
static bool multifd_zero_page_enabled(void)
{
return migrate_zero_page_detection() == ZERO_PAGE_DETECTION_MULTIFD;
}
static void swap_page_offset(ram_addr_t *pages_offset, int a, int b)
{
ram_addr_t temp;
if (a == b) {
return;
}
temp = pages_offset[a];
pages_offset[a] = pages_offset[b];
pages_offset[b] = temp;
}
/**
* multifd_send_zero_page_detect: Perform zero page detection on all pages.
*
* Sorts normal pages before zero pages in p->pages->offset and updates
* p->pages->normal_num.
*
* @param p A pointer to the send params.
*/
void multifd_send_zero_page_detect(MultiFDSendParams *p)
{
MultiFDPages_t *pages = &p->data->u.ram;
RAMBlock *rb = pages->block;
int i = 0;
int j = pages->num - 1;
if (!multifd_zero_page_enabled()) {
pages->normal_num = pages->num;
goto out;
}
/*
* Sort the page offset array by moving all normal pages to
* the left and all zero pages to the right of the array.
*/
while (i <= j) {
uint64_t offset = pages->offset[i];
if (!buffer_is_zero(rb->host + offset, multifd_ram_page_size())) {
i++;
continue;
}
swap_page_offset(pages->offset, i, j);
ram_release_page(rb->idstr, offset);
j--;
}
pages->normal_num = i;
out:
stat64_add(&mig_stats.normal_pages, pages->normal_num);
stat64_add(&mig_stats.zero_pages, pages->num - pages->normal_num);
}
void multifd_recv_zero_page_process(MultiFDRecvParams *p)
{
for (int i = 0; i < p->zero_num; i++) {
void *page = p->host + 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());
}
if (!received) {
ramblock_recv_bitmap_set_offset(p->block, p->zero[i]);
}
}
}
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