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

[Andrei Vagin <avagin@gmail.com>]

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@google.com
> 
> > 
> >> +		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.