[PATCH v12 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

[Muhammad Usama Anjum]

This IOCTL, PAGEMAP_SCAN on pagemap file can be used to get and/or clear
the info about page table entries. The following operations are supported
in this ioctl:
- Get the information if the pages have been written-to (PAGE_IS_WRITTEN),
  file mapped (PAGE_IS_FILE), present (PAGE_IS_PRESENT) or swapped
  (PAGE_IS_SWAPPED).
- Find pages which have been written-to and write protect the pages
  (atomic PAGE_IS_WRITTEN + PAGEMAP_WP_ENGAGE)

This IOCTL can be extended to get information about more PTE bits.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
---
Changes in v12:
- Add hugetlb support to cover all memory types
- Merge "userfaultfd: Define dummy uffd_wp_range()" with this patch
- Review updates to the code

Changes in v11:
- Find written pages in a better way
- Fix a corner case (thanks Paul)
- Improve the code/comments
- remove ENGAGE_WP + ! GET operation
- shorten the commit message in favour of moving documentation to
  pagemap.rst

Changes in v10:
- move changes in tools/include/uapi/linux/fs.h to separate patch
- update commit message

Change in v8:
- Correct is_pte_uffd_wp()
- Improve readability and error checks
- Remove some un-needed code

Changes in v7:
- Rebase on top of latest next
- Fix some corner cases
- Base soft-dirty on the uffd wp async
- Update the terminologies
- Optimize the memory usage inside the ioctl

Changes in v6:
- Rename variables and update comments
- Make IOCTL independent of soft_dirty config
- Change masks and bitmap type to _u64
- Improve code quality

Changes in v5:
- Remove tlb flushing even for clear operation

Changes in v4:
- Update the interface and implementation

Changes in v3:
- Tighten the user-kernel interface by using explicit types and add more
  error checking

Changes in v2:
- Convert the interface from syscall to ioctl
- Remove pidfd support as it doesn't make sense in ioctl
---
 fs/proc/task_mmu.c            | 426 ++++++++++++++++++++++++++++++++++
 include/linux/userfaultfd_k.h |   8 +
 include/uapi/linux/fs.h       |  53 +++++
 3 files changed, 487 insertions(+)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 6134f0df4164..36531b04bcd6 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -19,6 +19,7 @@
 #include <linux/shmem_fs.h>
 #include <linux/uaccess.h>
 #include <linux/pkeys.h>
+#include <linux/minmax.h>
 
 #include <asm/elf.h>
 #include <asm/tlb.h>
@@ -1135,6 +1136,26 @@ static inline void clear_soft_dirty(struct vm_area_struct *vma,
 }
 #endif
 
+static inline bool is_pte_uffd_wp(pte_t pte)
+{
+	return ((pte_present(pte) && pte_uffd_wp(pte)) ||
+		pte_swp_uffd_wp_any(pte));
+}
+
+#ifdef CONFIG_HUGETLB_PAGE
+static inline bool is_huge_pte_uffd_wp(pte_t pte)
+{
+	return ((pte_present(pte) && huge_pte_uffd_wp(pte)) ||
+		pte_swp_uffd_wp_any(pte));
+}
+#endif
+
+static inline bool is_pmd_uffd_wp(pmd_t pmd)
+{
+	return ((pmd_present(pmd) && pmd_uffd_wp(pmd)) ||
+		(is_swap_pmd(pmd) && pmd_swp_uffd_wp(pmd)));
+}
+
 #if defined(CONFIG_MEM_SOFT_DIRTY) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
 static inline void clear_soft_dirty_pmd(struct vm_area_struct *vma,
 		unsigned long addr, pmd_t *pmdp)
@@ -1768,11 +1789,416 @@ static int pagemap_release(struct inode *inode, struct file *file)
 	return 0;
 }
 
+#define PM_SCAN_FOUND_MAX_PAGES	(1)
+#define PM_SCAN_BITS_ALL	(PAGE_IS_WRITTEN | PAGE_IS_FILE |	\
+				 PAGE_IS_PRESENT | PAGE_IS_SWAPPED)
+#define PM_SCAN_OPS		(PM_SCAN_OP_GET | PM_SCAN_OP_WP)
+#define PM_SCAN_OP_IS_WP(a)	(a->flags & PM_SCAN_OP_WP)
+#define PM_SCAN_BITMAP(wt, file, present, swap)	\
+	(wt | file << 1 | present << 2 | swap << 3)
+
+struct pagemap_scan_private {
+	struct page_region *vec;
+	struct page_region cur;
+	unsigned long vec_len, vec_index;
+	unsigned int max_pages, found_pages, flags;
+	unsigned long required_mask, anyof_mask, excluded_mask, return_mask;
+};
+
+static inline bool pagemap_scan_is_written_set(struct pagemap_scan_private *p)
+{
+	return	((p->required_mask | p->anyof_mask | p->excluded_mask) &
+		 PAGE_IS_WRITTEN);
+}
+
+static int pagemap_scan_test_walk(unsigned long start, unsigned long end,
+				  struct mm_walk *walk)
+{
+	struct pagemap_scan_private *p = walk->private;
+	struct vm_area_struct *vma = walk->vma;
+
+	if (pagemap_scan_is_written_set(p) && (!userfaultfd_wp(vma) ||
+	    !userfaultfd_wp_async(vma)))
+		return -EPERM;
+
+	if (vma->vm_flags & VM_PFNMAP)
+		return 1;
+
+	return 0;
+}
+
+static int pagemap_scan_output(bool wt, bool file, bool pres, bool swap,
+			       struct pagemap_scan_private *p,
+			       unsigned long addr, unsigned int n_pages)
+{
+	unsigned long bitmap = PM_SCAN_BITMAP(wt, file, pres, swap);
+	struct page_region *cur = &p->cur;
+
+	if (!n_pages)
+		return -EINVAL;
+
+	if ((p->required_mask & bitmap) != p->required_mask)
+		return 0;
+	if (p->anyof_mask && !(p->anyof_mask & bitmap))
+		return 0;
+	if (p->excluded_mask & bitmap)
+		return 0;
+
+	bitmap &= p->return_mask;
+	if (!bitmap)
+		return 0;
+
+	if ((cur->start + cur->len * PAGE_SIZE == addr) &&
+	    (cur->bitmap == bitmap)) {
+		cur->len += n_pages;
+		p->found_pages += n_pages;
+
+		if (p->max_pages && (p->found_pages == p->max_pages))
+			return PM_SCAN_FOUND_MAX_PAGES;
+
+		return 0;
+	}
+
+	if (!p->vec_index || ((p->vec_index + 1) < p->vec_len)) {
+
+		if (cur->len) {
+			memcpy(&p->vec[p->vec_index], cur,
+			       sizeof(struct page_region));
+			p->vec_index++;
+		}
+		cur->start = addr;
+		cur->len = n_pages;
+		cur->bitmap = bitmap;
+		p->found_pages += n_pages;
+
+		if (p->max_pages && (p->found_pages == p->max_pages))
+			return PM_SCAN_FOUND_MAX_PAGES;
+
+		return 0;
+	}
+
+	return -ENOSPC;
+}
+
+static inline int pagemap_scan_deposit(struct pagemap_scan_private *p,
+				       struct page_region __user *vec,
+				       unsigned long *vec_index)
+{
+	struct page_region *cur = &p->cur;
+
+	if (!cur->len)
+		return 0;
+
+	if (copy_to_user(&vec[*vec_index], cur, sizeof(struct page_region)))
+		return -EFAULT;
+
+	p->vec_index++;
+	(*vec_index)++;
+
+	return 0;
+}
+
+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;
+	bool is_written, is_file, is_present, is_swap;
+	struct vm_area_struct *vma = walk->vma;
+	unsigned long addr = end;
+	spinlock_t *ptl;
+	int ret = 0;
+	pte_t *pte;
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	ptl = pmd_trans_huge_lock(pmd, vma);
+	if (ptl) {
+		unsigned long n_pages = (end - start)/PAGE_SIZE;
+
+		is_written = !is_pmd_uffd_wp(*pmd);
+		is_file = vma->vm_file;
+		is_present = pmd_present(*pmd);
+		is_swap = is_swap_pmd(*pmd);
+
+		spin_unlock(ptl);
+
+		/*
+		 * Break huge page into small pages if the WP operation need to
+		 * be performed is on a portion of the huge page or if max_pages
+		 * pages limit would exceed.
+		 */
+		if (is_written && PM_SCAN_OP_IS_WP(p) &&
+		    ((end - start < HPAGE_SIZE) ||
+		     (p->max_pages &&
+		      (p->max_pages - p->found_pages) < n_pages))) {
+
+			split_huge_pmd(vma, pmd, start);
+			goto process_smaller_pages;
+		}
+
+		if (p->max_pages &&
+		    p->found_pages + n_pages > p->max_pages)
+			n_pages = p->max_pages - p->found_pages;
+
+		ret = pagemap_scan_output(is_written, is_file, is_present,
+					  is_swap, p, start, n_pages);
+		if (ret < 0)
+			return ret;
+
+		if (is_written && PM_SCAN_OP_IS_WP(p) &&
+		    uffd_wp_range(vma, start, HPAGE_SIZE, true) < 0)
+			ret = -EINVAL;
+
+		return ret;
+	}
+process_smaller_pages:
+	if (pmd_trans_unstable(pmd))
+		return 0;
+#endif
+
+	for (addr = start; addr < end && !ret; pte++, addr += PAGE_SIZE) {
+		pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
+
+		is_written = !is_pte_uffd_wp(*pte);
+		is_file = vma->vm_file;
+		is_present = pte_present(*pte);
+		is_swap = is_swap_pte(*pte);
+
+		pte_unmap_unlock(pte, ptl);
+
+		ret = pagemap_scan_output(is_written, is_file, is_present,
+					  is_swap, p, addr, 1);
+		if (ret < 0)
+			return ret;
+
+		if (is_written && PM_SCAN_OP_IS_WP(p) &&
+		    uffd_wp_range(vma, addr, PAGE_SIZE, true) < 0)
+			return -EINVAL;
+	}
+
+	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)
+{
+	struct pagemap_scan_private *p = walk->private;
+	bool is_written;
+	struct vm_area_struct *vma = walk->vma;
+	unsigned long n_pages = (end - start)/PAGE_SIZE;
+	pte_t pte;
+	int ret;
+
+	pte = huge_ptep_get(ptep);
+	is_written = !is_huge_pte_uffd_wp(pte);
+
+	/*
+	 * Partial hugetlb page clear isn't supported
+	 */
+	if (is_written && PM_SCAN_OP_IS_WP(p) &&
+	    ((end - start < HPAGE_SIZE) ||
+	     (p->max_pages &&
+	      (p->max_pages - p->found_pages) < n_pages)))
+		return -EPERM;
+
+	if (p->max_pages &&
+	    p->found_pages + n_pages > p->max_pages)
+		n_pages = p->max_pages - p->found_pages;
+
+	ret = pagemap_scan_output(is_written, vma->vm_file, pte_present(pte),
+				  is_swap_pte(pte), p, start, n_pages);
+	if (ret < 0)
+		return ret;
+
+	if (is_written && PM_SCAN_OP_IS_WP(p) &&
+	    uffd_wp_range(vma, start, HPAGE_SIZE, true) < 0)
+		ret = -EINVAL;
+
+	return ret;
+}
+#else
+#define pagemap_scan_hugetlb_entry NULL
+#endif
+
+static int pagemap_scan_pte_hole(unsigned long addr, unsigned long end,
+				 int depth, struct mm_walk *walk)
+{
+	struct pagemap_scan_private *p = walk->private;
+	struct vm_area_struct *vma = walk->vma;
+	unsigned long n_pages;
+	int ret = 0;
+
+	if (!vma)
+		return 0;
+
+	n_pages = (end - addr)/PAGE_SIZE;
+	if (p->max_pages && p->found_pages + n_pages > p->max_pages)
+		n_pages = p->max_pages - p->found_pages;
+
+	ret = pagemap_scan_output(false, vma->vm_file, false, false, p, addr,
+				  n_pages);
+
+	return ret;
+}
+
+static const struct mm_walk_ops pagemap_scan_ops = {
+	.test_walk = pagemap_scan_test_walk,
+	.pmd_entry = pagemap_scan_pmd_entry,
+	.pte_hole = pagemap_scan_pte_hole,
+	.hugetlb_entry = pagemap_scan_hugetlb_entry,
+};
+
+static int pagemap_scan_args_valid(struct pm_scan_arg *arg, unsigned long start,
+				   struct page_region __user *vec)
+{
+	/* Detect illegal size, flags, len and masks */
+	if (arg->size != sizeof(struct pm_scan_arg))
+		return -EINVAL;
+	if (arg->flags & ~PM_SCAN_OPS)
+		return -EINVAL;
+	if (!arg->len)
+		return -EINVAL;
+	if ((arg->required_mask | arg->anyof_mask | arg->excluded_mask |
+	     arg->return_mask) & ~PM_SCAN_BITS_ALL)
+		return -EINVAL;
+	if (!arg->required_mask && !arg->anyof_mask &&
+	    !arg->excluded_mask)
+		return -EINVAL;
+	if (!arg->return_mask)
+		return -EINVAL;
+
+	/* Validate memory ranges */
+	if (!(arg->flags & PM_SCAN_OP_GET))
+		return -EINVAL;
+	if (!arg->vec)
+		return -EINVAL;
+	if (arg->vec_len == 0)
+		return -EINVAL;
+
+	if (!IS_ALIGNED(start, PAGE_SIZE))
+		return -EINVAL;
+	if (!access_ok((void __user *)start, arg->len))
+		return -EFAULT;
+
+	if (!PM_SCAN_OP_IS_WP(arg))
+		return 0;
+
+	if ((arg->required_mask | arg->anyof_mask | arg->excluded_mask) &
+	    ~PAGE_IS_WRITTEN)
+		return -EINVAL;
+
+	return 0;
+}
+
+static long do_pagemap_cmd(struct mm_struct *mm,
+			   struct pm_scan_arg __user *uarg)
+{
+	unsigned long start, end, walk_start, walk_end;
+	unsigned long empty_slots, vec_index = 0;
+	struct page_region __user *vec;
+	struct pagemap_scan_private p;
+	struct pm_scan_arg arg;
+	int ret = 0;
+
+	if (copy_from_user(&arg, uarg, sizeof(arg)))
+		return -EFAULT;
+
+	start = (unsigned long)untagged_addr(arg.start);
+	vec = (struct page_region *)(unsigned long)untagged_addr(arg.vec);
+
+	ret = pagemap_scan_args_valid(&arg, start, vec);
+	if (ret)
+		return ret;
+
+	end = start + arg.len;
+	p.max_pages = arg.max_pages;
+	p.found_pages = 0;
+	p.flags = arg.flags;
+	p.required_mask = arg.required_mask;
+	p.anyof_mask = arg.anyof_mask;
+	p.excluded_mask = arg.excluded_mask;
+	p.return_mask = arg.return_mask;
+	p.cur.len = 0;
+	p.cur.start = 0;
+	p.vec = NULL;
+	p.vec_len = (PAGEMAP_WALK_SIZE >> PAGE_SHIFT);
+
+	/*
+	 * Allocate smaller buffer to get output from inside the page walk
+	 * functions and walk page range in PAGEMAP_WALK_SIZE size chunks. As
+	 * we want to return output to user in compact form where no two
+	 * consecutive regions should be continuous and have the same flags.
+	 * So store the latest element in p.cur between different walks and
+	 * store the p.cur at the end of the walk to the user buffer.
+	 */
+	p.vec = kmalloc_array(p.vec_len, sizeof(struct page_region),
+			      GFP_KERNEL);
+	if (!p.vec)
+		return -ENOMEM;
+
+	walk_start = walk_end = start;
+	while (walk_end < end && !ret) {
+		p.vec_index = 0;
+
+		empty_slots = arg.vec_len - vec_index;
+		p.vec_len = min(p.vec_len, empty_slots);
+
+		walk_end = (walk_start + PAGEMAP_WALK_SIZE) & PAGEMAP_WALK_MASK;
+		if (walk_end > end)
+			walk_end = end;
+
+		mmap_read_lock(mm);
+		ret = walk_page_range(mm, walk_start, walk_end,
+				      &pagemap_scan_ops, &p);
+		mmap_read_unlock(mm);
+
+		if (ret && ret != -ENOSPC && ret != PM_SCAN_FOUND_MAX_PAGES)
+			goto free_data;
+
+		walk_start = walk_end;
+		if (p.vec_index) {
+			if (copy_to_user(&vec[vec_index], p.vec,
+					 p.vec_index *
+					 sizeof(struct page_region))) {
+				ret = -EFAULT;
+				goto free_data;
+			}
+			vec_index += p.vec_index;
+		}
+	}
+	ret = pagemap_scan_deposit(&p, vec, &vec_index);
+	if (!ret)
+		ret = vec_index;
+free_data:
+	kfree(p.vec);
+
+	return ret;
+}
+
+static long pagemap_scan_ioctl(struct file *file, unsigned int cmd,
+			       unsigned long arg)
+{
+	struct pm_scan_arg __user *uarg = (struct pm_scan_arg __user *)arg;
+	struct mm_struct *mm = file->private_data;
+
+	switch (cmd) {
+	case PAGEMAP_SCAN:
+		return do_pagemap_cmd(mm, uarg);
+
+	default:
+		return -EINVAL;
+	}
+}
+
 const struct file_operations proc_pagemap_operations = {
 	.llseek		= mem_lseek, /* borrow this */
 	.read		= pagemap_read,
 	.open		= pagemap_open,
 	.release	= pagemap_release,
+	.unlocked_ioctl = pagemap_scan_ioctl,
+	.compat_ioctl	= pagemap_scan_ioctl,
 };
 #endif /* CONFIG_PROC_PAGE_MONITOR */
 
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index ce4f9d18de3e..325672cd6d6f 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -210,6 +210,14 @@ extern bool userfaultfd_wp_async(struct vm_area_struct *vma);
 
 #else /* CONFIG_USERFAULTFD */
 
+static inline long uffd_wp_range(struct mm_struct *dst_mm,
+				 struct vm_area_struct *vma,
+				 unsigned long start, unsigned long len,
+				 bool enable_wp)
+{
+	return 0;
+}
+
 /* mm helpers */
 static inline vm_fault_t handle_userfault(struct vm_fault *vmf,
 				unsigned long reason)
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index b7b56871029c..47879c38ce2f 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -305,4 +305,57 @@ typedef int __bitwise __kernel_rwf_t;
 #define RWF_SUPPORTED	(RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
 			 RWF_APPEND)
 
+/* Pagemap ioctl */
+#define PAGEMAP_SCAN	_IOWR('f', 16, struct pm_scan_arg)
+
+/* Bits are set in the bitmap of the page_region and masks in pm_scan_args */
+#define PAGE_IS_WRITTEN		(1 << 0)
+#define PAGE_IS_FILE		(1 << 1)
+#define PAGE_IS_PRESENT		(1 << 2)
+#define PAGE_IS_SWAPPED		(1 << 3)
+
+/*
+ * struct page_region - Page region with bitmap flags
+ * @start:	Start of the region
+ * @len:	Length of the region in pages
+ * bitmap:	Bits sets for the region
+ */
+struct page_region {
+	__u64 start;
+	__u64 len;
+	__u64 bitmap;
+};
+
+/*
+ * struct pm_scan_arg - Pagemap ioctl argument
+ * @size:		Size of the structure
+ * @flags:		Flags for the IOCTL
+ * @start:		Starting address of the region
+ * @len:		Length of the region (All the pages in this length are included)
+ * @vec:		Address of page_region struct array for output
+ * @vec_len:		Length of the page_region struct array
+ * @max_pages:		Optional max return pages
+ * @required_mask:	Required mask - All of these bits have to be set in the PTE
+ * @anyof_mask:		Any mask - Any of these bits are set in the PTE
+ * @excluded_mask:	Exclude mask - None of these bits are set in the PTE
+ * @return_mask:	Bits that are to be reported in page_region
+ */
+struct pm_scan_arg {
+	__u64 size;
+	__u64 flags;
+	__u64 start;
+	__u64 len;
+	__u64 vec;
+	__u64 vec_len;
+	__u64 max_pages;
+	__u64 required_mask;
+	__u64 anyof_mask;
+	__u64 excluded_mask;
+	__u64 return_mask;
+};
+
+/* Supported flags */
+#define PM_SCAN_OP_GET	(1 << 0)
+#define PM_SCAN_OP_WP	(1 << 1)
+
 #endif /* _UAPI_LINUX_FS_H */
-- 
2.39.2