Re: [PATCH v16 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

On 6/1/23 2:46 AM, Peter Xu wrote:

Muhammad,

Sorry, I probably can only review the non-interface part, and leave the interface/buffer handling, etc. review for others and real potential users of it..

Thank you so much for the review. I think mostly we should be okay with interface as everybody has been making suggestions over the past revisions.

On Thu, May 25, 2023 at 01:55:14PM +0500, Muhammad Usama Anjum wrote:

...

+static inline void make_uffd_wp_huge_pte(struct vm_area_struct *vma,

• 			 unsigned long addr, pte_t *ptep,
  

• 			 pte_t ptent)
  

+{

• pte_t old_pte;
• if (!huge_pte_none(ptent)) {

• old_pte = huge_ptep_modify_prot_start(vma, addr, ptep);
  

• ptent = huge_pte_mkuffd_wp(old_pte);
  

• ptep_modify_prot_commit(vma, addr, ptep, old_pte, ptent);
  

huge_ptep_modify_prot_start()?

Sorry, I didn't realized that huge_ptep_modify_prot_start() is different from its pte version.

The other thing is what if it's a pte marker already? What if a hugetlb migration entry? Please check hugetlb_change_protection().

I've updated it in more better way. Please let me know what do you think about the following:

static inline void make_uffd_wp_huge_pte(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep, pte_t ptent) { if (is_hugetlb_entry_hwpoisoned(ptent) || is_pte_marker(ptent)) return;

if (is_hugetlb_entry_migration(ptent)) set_huge_pte_at(vma->vm_mm, addr, ptep, pte_swp_mkuffd_wp(ptent)); else if (!huge_pte_none(ptent)) ptep_modify_prot_commit(vma, addr, ptep, ptent, huge_pte_mkuffd_wp(ptent)); else set_huge_pte_at(vma->vm_mm, addr, ptep, make_pte_marker(PTE_MARKER_UFFD_WP)); }

As we always set UNPOPULATED, so markers are always set on none ptes initially. Is it possible that a none pte becomes present, then swapped and finally none again? So I'll do the following addition for make_uffd_wp_pte():

--- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -1800,6 +1800,9 @@ static inline void make_uffd_wp_pte(struct vm_area_struct *vma, } else if (is_swap_pte(ptent)) { ptent = pte_swp_mkuffd_wp(ptent); set_pte_at(vma->vm_mm, addr, pte, ptent); + } else { + set_pte_at(vma->vm_mm, addr, pte, + make_pte_marker(PTE_MARKER_UFFD_WP)); } }

...

• } else {

• set_huge_pte_at(vma->vm_mm, addr, ptep,
  

• 		make_pte_marker(PTE_MARKER_UFFD_WP));
  

• }

+} +#endif

[...]

...

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

• 		  unsigned long end, struct mm_walk *walk)
  

+{

• struct pagemap_scan_private *p = walk->private;
• struct vm_area_struct *vma = walk->vma;
• unsigned long addr = end;
• pte_t *pte, *orig_pte;
• spinlock_t *ptl;
• bool is_written;
• 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;
  

• }
  

• if (IS_PM_SCAN_GET(p->flags))
  

• 	ret = pagemap_scan_output(is_written, vma->vm_file,
  

• 				  pmd_present(*pmd),
  

• 				  is_swap_pmd(*pmd),
  

• 				  p, start, n_pages);
  

• if (ret >= 0 && is_written && IS_PM_SCAN_WP(p->flags))
  

• 	make_uffd_wp_pmd(vma, addr, pmd);
  

• if (IS_PM_SCAN_WP(p->flags))
  

• 	flush_tlb_range(vma, start, end);
  

• spin_unlock(ptl);
  

• arch_leave_lazy_mmu_mode();
  

• return ret;
  

• }

+process_smaller_pages:

• if (pmd_trans_unstable(pmd)) {

• arch_leave_lazy_mmu_mode();
  

• return 0;
  

I'm not sure whether this is right.. Shouldn't you return with -EAGAIN and let the user retry? Returning 0 means you'll move on with the next pmd afaict and ignoring this one.

This has come up before. We are just replicating pagemap_pmd_range() here as we are doing almost the same thing through IOCTL. It doesn't return any error in this case and just skips it. So we are doing the same.

...

• }

+#endif

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

Just a heads-up that this may start to fail at some point if Hugh's work will land earlier:

https://lore.kernel.org/linux-mm/68a97fbe-5c1e-7ac6-72c-7b9c6290b370@google....

Thank you so much for the heads up.

...

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

• is_written = !is_pte_uffd_wp(*pte);
  

• if (IS_PM_SCAN_GET(p->flags))
  

• 	ret = pagemap_scan_output(is_written, vma->vm_file,
  

• 				  pte_present(*pte),
  

• 				  is_swap_pte(*pte),
  

• 				  p, addr, 1);
  

• if (ret >= 0 && is_written && IS_PM_SCAN_WP(p->flags))
  

• 	make_uffd_wp_pte(vma, addr, pte);
  

• }
• if (IS_PM_SCAN_WP(p->flags))

• flush_tlb_range(vma, start, addr);
  

• pte_unmap_unlock(orig_pte, ptl);
• arch_leave_lazy_mmu_mode();
• cond_resched();
• return ret;

+}

+#ifdef CONFIG_HUGETLB_PAGE +static int pagemap_scan_hugetlb_entry(pte_t *ptep, unsigned long hmask,

• 		      unsigned long start, unsigned long end,
  

• 		      struct mm_walk *walk)
  

+{

• unsigned long n_pages = (end - start)/PAGE_SIZE;
• struct pagemap_scan_private *p = walk->private;
• struct vm_area_struct *vma = walk->vma;
• struct hstate *h = hstate_vma(vma);
• spinlock_t *ptl;
• bool is_written;
• int ret = 0;
• pte_t pte;
• arch_enter_lazy_mmu_mode();

This _seems_ to be not needed for hugetlb entries.

I'll remove it.

...

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

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

• if (IS_PM_SCAN_WP(p->flags)) {

• i_mmap_lock_write(vma->vm_file->f_mapping);
  

• ptl = huge_pte_lock(h, vma->vm_mm, ptep);
  

• }
• pte = huge_ptep_get(ptep);
• is_written = !is_huge_pte_uffd_wp(pte);
• /*

• * Partial hugetlb page clear isn't supported
  

• */
  

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

•    n_pages < HPAGE_SIZE/PAGE_SIZE) {
  

• ret = -EPERM;
  

• goto unlock_and_return;
  

• }
• if (IS_PM_SCAN_GET(p->flags)) {

• ret = pagemap_scan_output(is_written, vma->vm_file,
  

• 			  pte_present(pte), is_swap_pte(pte),
  

• 			  p, start, n_pages);
  

• if (ret < 0)
  

• 	goto unlock_and_return;
  

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

• make_uffd_wp_huge_pte(vma, start, ptep, pte);
  

• flush_hugetlb_tlb_range(vma, start, end);
  

• }

+unlock_and_return:

• if (IS_PM_SCAN_WP(p->flags)) {

• spin_unlock(ptl);
  

• i_mmap_unlock_write(vma->vm_file->f_mapping);
  

• }
• arch_leave_lazy_mmu_mode();

Same here.

...

• return ret;

+}

[...]