On Mon, Jun 19, 2023 at 11:06:36AM +0500, Muhammad Usama Anjum wrote:
> On 6/17/23 11:39 AM, Andrei Vagin wrote:
> > 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);
> No, please look at these recently merged patches:
> https://lore.kernel.org/all/c1c9a74a-bc5b-15ea-e5d2-8ec34bc921d@xxxxxxxxxx
>
> >
> >> + 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'll update.
>
> >
> > 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.
> Should we name it ZERO_PFN_PRESENT_PAGE to be exact or what?
IMHO, ZERO_PFN_PRESENT_PAGE looks a bit monstrous.
It looks like zero page is a proper noun in the kernel, so PAGE_IS_ZERO
might be a good choice here, but it is up to you.
>
> >
> > 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)));
> Okay. Can you please confirm my assumptions:
> - A THP cannot be file backed. (PM_FILE isn't being set for THP case)
```
Currently THP only works for anonymous memory mappings and tmpfs/shmem.
But in the future it can expand to other filesystems.
```
https://www.kernel.org/doc/html/next/admin-guide/mm/transhuge.html
so THP can be "file backed".
> - A hole is also not file backed.
>
> A hole isn't present in memory. So its pfn would be zero. But as it isn't
> present, it shouldn't report zero page. Right? For hole::
>
> PM_SCAN_FLAGS(false, false, false, false, false)
This looks correct to me.
