“PE_LAZY” flag in parent pagemap images?

[obsidian0215]

Description

I tried to implement a hybrid live-migration using CRIU, so I used criu pre-dump to migrate part of process's memory like post-copy. But when I executed criu dump --lazy-pages and criu restore --lazy-pages with criu lazy-pages as post-copy, I found that size of memory that migrated by uffd is almost equal to the memory size when I only use post-copy, which means pre-copy cannot affect(reduce) transmission of post-copy.
In order to identify the cause of this phenomenon, I dived into CRIU's source code and found that CRIU skips pagemap entrys with PE_LAZY during memory's lazy-restore. However, many of present pages in the parent pagemap images are also marked as PE_LAZY, resulting in almost no utilization of parent page images when restoring. So I have a question about the meaning of adding the PE_LAZY flag to the pages selected by pre-dump - the pages selected by dump can skip writing to the image, transferring, and restoring through PE_LAZY, but the pre dump pages have already been written to the image and transferred, and PE_LAZY seems to just skip restoring. This actually increases unnecessary memory transfer, so I believe it's better to avoid PE_LAZY for pagemaps in pre-dump. If this idea is incorrect, I would like to know the reason behind it. Thank you.

Steps to reproduce the issue:

1. criu pre-dump ... for several iterations and transfer the pre-dumped memory
2. criu dump --lazy-pages ... and transfer the dumped states
3. criu restore --lazy-pages with criu lazy-pages to restore the process

Describe the results you received:
The memory size transmitted by Lazy-Pages is not affected by Pre-Dump

Describe the results you expected:
The memory size transmitted by Lazy-Pages may be reduced by Pre-Dump

Additional information you deem important (e.g. issue happens only occasionally):
in criu/mem.c:

static int generate_iovs(struct pstree_item *item, struct vma_area *vma, struct page_pipe *pp, pmc_t *pmc, u64 *pvaddr,
			 bool has_parent)
{
    ...
    if (vma_entry_can_be_lazy(vma->e) && !is_stack(item, vaddr))
          ppb_flags |= PPB_LAZY;
    ...
}

(PPB_LAZY will be transformed to PE_LAZY when generating pagemap.)

static int restore_priv_vma_content(struct pstree_item *t, struct page_read *pr)
{
        ...
	/*
	 * Read page contents.
	 */
	while (1) {
		unsigned long off, i, nr_pages;

		ret = pr->advance(pr);
		if (ret <= 0)
			break;

		va = (unsigned long)decode_pointer(pr->pe->vaddr);
		nr_pages = pr->pe->nr_pages;

		/*
		 * This means that userfaultfd is used to load the pages
		 * on demand.
		 */
		if (opts.lazy_pages && pagemap_lazy(pr->pe)) {
			pr_debug("Lazy restore skips %ld pages at %lx\n", nr_pages, va);
			pr->skip_pages(pr, nr_pages * PAGE_SIZE);
			nr_lazy += nr_pages;
			continue;
		}
          ...
}

CRIU logs and information:

CRIU full dump/restore logs:

NULL

Output of `criu --version`:

4.0

Output of `criu check --all`:

Looks good.

Additional environment details: