On 7/1/21 6:46 AM, Kaiyang Zhao wrote:
> The design is that instead of copying the entire paging tree during the
> fork invocation, we make the child and the parent process share the same
> set of last-level page tables, which will be reference counted. To preserve
> the copy-on-write semantics, we disable the write permission in PMD entries
> in fork, and copy PTE tables as needed in the page fault handler.
That's clever. But, I'm not sure it's comprehensive. How, for
instance, do you handle get_user_pages() users that don't actually write
to the mappings? Or, memory reclaim where the kernel itself goes and
zaps page table entries without accessing the mapping itself that's
being zapped.
I would have expected a *lot* more pervasive changes to page table
walkers across the kernel.
Oh, and the code itself makes my eyes bleed. You might want to spend a
bit of time to clean out the debug printk()s and make sure this gets
somewhere close to passing checkpatch.pl if you want to be taken more
seriously. For example:
> + if (pte_present(ptent)) {
> + struct page *page;
> +
> + if (pte_special(ptent)) { //known special pte: vvar VMA, which has just one page shared system-wide. Shouldn't matter
> + continue;
> + }
> + page = vm_normal_page(NULL, addr, ptent); //kyz : vma is not important
> + if (unlikely(!page))
> + continue;
> + rss[mm_counter(page)]--;
> +#ifdef CONFIG_DEBUG_VM
> + // printk("zap_one_pte_table: addr=%lx, end=%lx, (before) mapcount=%d, refcount=%d\n", addr, end, page_mapcount(page), page_ref_count(page));
> +#endif
> + page_remove_rmap(page, false);
> + put_page(page);
> + }
