Re: [PATCH v19 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

On Thu, Jun 15, 2023 at 07:11:41PM +0500, Muhammad Usama Anjum wrote:

+static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,

• 		  unsigned long end, struct mm_walk *walk)
  

+{

• bool is_written, flush = false, is_interesting = true;
• struct pagemap_scan_private *p = walk->private;
• struct vm_area_struct *vma = walk->vma;
• unsigned long bitmap, addr = end;
• pte_t *pte, *orig_pte, ptent;
• spinlock_t *ptl;
• int ret = 0;
• arch_enter_lazy_mmu_mode();

+#ifdef CONFIG_TRANSPARENT_HUGEPAGE

• ptl = pmd_trans_huge_lock(pmd, vma);
• if (ptl) {

• unsigned long n_pages = (end - start)/PAGE_SIZE;
  

• if (p->max_pages && n_pages > p->max_pages - p->found_pages)
  

• 	n_pages = p->max_pages - p->found_pages;
  

• is_written = !is_pmd_uffd_wp(*pmd);
  

• /*
  

•  * Break huge page into small pages if the WP operation need to
  

•  * be performed is on a portion of the huge page.
  

•  */
  

• if (is_written && IS_PM_SCAN_WP(p->flags) &&
  

•     n_pages < HPAGE_SIZE/PAGE_SIZE) {
  

• 	spin_unlock(ptl);
  

• 	split_huge_pmd(vma, pmd, start);
  

• 	goto process_smaller_pages;
  

• }
  

• bitmap = PM_SCAN_FLAGS(is_written, (bool)vma->vm_file,
  

• 		       pmd_present(*pmd), is_swap_pmd(*pmd));
  

• if (IS_PM_SCAN_GET(p->flags)) {
  

• 	is_interesting = pagemap_scan_is_interesting_page(bitmap, p);
  

• 	if (is_interesting)
  

• 		ret = pagemap_scan_output(bitmap, p, start, n_pages);
  

• }
  

• if (IS_PM_SCAN_WP(p->flags) && is_written && is_interesting &&
  

•     ret >= 0) {
  

• 	make_uffd_wp_pmd(vma, start, pmd);
  

• 	flush_tlb_range(vma, start, end);
  

• }
  

• spin_unlock(ptl);
  

• arch_leave_lazy_mmu_mode();
  

• return ret;
  

• }

+process_smaller_pages: +#endif

• orig_pte = pte = pte_offset_map_lock(vma->vm_mm, pmd, start, &ptl);
• if (!pte) {

Do we need to unlock ptl here?

spin_unlock(ptl);

• walk->action = ACTION_AGAIN;
  

• return 0;
  

• }
• for (addr = start; addr < end && !ret; pte++, addr += PAGE_SIZE) {

• ptent = ptep_get(pte);
  

• is_written = !is_pte_uffd_wp(ptent);
  

• bitmap = PM_SCAN_FLAGS(is_written, (bool)vma->vm_file,
  

• 		       pte_present(ptent), is_swap_pte(ptent));
  

The vma->vm_file check isn't correct in this case. You can look when pte_to_pagemap_entry sets PM_FILE. This flag is used to detect what pages have a file backing store and what pages are anonymous.

I was trying to integrate this new interace into CRIU and I found one more thing that is required. We need to detect zero pages.

It should look something like this:

#define PM_SCAN_FLAGS(wt, file, present, swap, zero) \ ((wt) | ((file) << 1) | ((present) << 2) | ((swap) << 3) | ((zero) << 4))

bitmap = PM_SCAN_FLAGS(is_written, page && !PageAnon(page), pte_present(ptent), is_swap_pte(ptent), pte_present(ptent) && is_zero_pfn(pte_pfn(ptent)));

Thanks, Andrei