[PATCH v7 3/4] fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about PTEs

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

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_Wt),
  file mapped (PAGE_IS_FILE), present (PAGE_IS_PRESENT) or swapped
  (PAGE_IS_SWAPPED).
- Write-protect the pages (PAGEMAP_WP_ENGAGE) to start finding which
  pages have been written-to.
- Find pages which have been written-to and write protect the pages
  (atomic PAGE_IS_NOT_WP + PAGEMAP_WP_ENGAGE)

The uffd should have been registered on the memory range before performing
any WP/WT (Write Protect/Writtern-To) related operations with the IOCTL.

struct pagemap_scan_args is used as the argument of the IOCTL. In this
struct:
- The range is specified through start and len.
- The output buffer and size is specified as vec and vec_len.
- The optional maximum requested pages are specified in the max_pages.
- The flags can be specified in the flags field. The PAGEMAP_WP_ENGAGE
  is the only added flag at this time.
- The masks are specified in required_mask, anyof_mask, excluded_ mask
  and return_mask.

This IOCTL can be extended to get information about more PTE bits. This
patch has evolved from a basic patch from Gabriel Krisman Bertazi.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
---
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            | 300 ++++++++++++++++++++++++++++++++++
 include/uapi/linux/fs.h       |  50 ++++++
 tools/include/uapi/linux/fs.h |  50 ++++++
 3 files changed, 400 insertions(+)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index e35a0398db63..ba70faadf403 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 +1136,22 @@ static inline void clear_soft_dirty(struct vm_area_struct *vma,
 }
 #endif
 
+static inline bool is_pte_uffd_wp(pte_t pte)
+{
+	if ((pte_present(pte) && pte_uffd_wp(pte)) ||
+	    (is_swap_pte(pte) && pte_swp_uffd_wp(pte)))
+		return true;
+	return false;
+}
+
+static inline bool is_pmd_uffd_wp(pmd_t pmd)
+{
+	if ((pmd_present(pmd) && pmd_uffd_wp(pmd)) ||
+	    (is_swap_pmd(pmd) && pmd_swp_uffd_wp(pmd)))
+		return true;
+	return false;
+}
+
 #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)
@@ -1763,11 +1780,294 @@ static int pagemap_release(struct inode *inode, struct file *file)
 	return 0;
 }
 
+#define PAGEMAP_OP_MASK		(PAGE_IS_WT | PAGE_IS_FILE |	\
+				 PAGE_IS_PRESENT | PAGE_IS_SWAPPED)
+#define PAGEMAP_NONWT_OP_MASK	(PAGE_IS_FILE |	PAGE_IS_PRESENT | PAGE_IS_SWAPPED)
+#define IS_WP_ENGAGE_OP(a)	(a->flags & PAGEMAP_WP_ENGAGE)
+#define IS_GET_OP(a)		(a->vec)
+#define PAGEMAP_SCAN_BITMAP(wt, file, present, swap)	\
+	(wt | file << 1 | present << 2 | swap << 3)
+#define IS_WT_REQUIRED(a)				\
+	((a->required_mask & PAGE_IS_WT) ||	\
+	 (a->anyof_mask & PAGE_IS_WT))
+
+struct pagemap_scan_private {
+	struct page_region *vec;
+	struct page_region prev;
+	unsigned long vec_len, vec_index;
+	unsigned int max_pages, found_pages, flags;
+	unsigned long required_mask, anyof_mask, excluded_mask, return_mask;
+};
+
+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 (IS_WT_REQUIRED(p) && !vma_can_userfault(vma, vma->vm_flags))
+		return -EPERM;
+	if (IS_GET_OP(p) && p->max_pages && (p->found_pages == p->max_pages))
+		return -ENOSPC;
+	if (vma->vm_flags & VM_PFNMAP)
+		return 1;
+	return 0;
+}
+
+static inline int add_to_out(bool wt, bool file, bool pres, bool swap,
+			     struct pagemap_scan_private *p, unsigned long addr, unsigned int len)
+{
+	unsigned long bitmap, cur = PAGEMAP_SCAN_BITMAP(wt, file, pres, swap);
+	bool cpy = true;
+	struct page_region *prev = &p->prev;
+
+	if (p->required_mask)
+		cpy = ((p->required_mask & cur) == p->required_mask);
+	if (cpy && p->anyof_mask)
+		cpy = (p->anyof_mask & cur);
+	if (cpy && p->excluded_mask)
+		cpy = !(p->excluded_mask & cur);
+	bitmap = cur & p->return_mask;
+	if (cpy && bitmap) {
+		if ((prev->len) && (prev->bitmap == bitmap) &&
+		    (prev->start + prev->len * PAGE_SIZE == addr)) {
+			prev->len += len;
+			p->found_pages += len;
+		} else if (p->vec_index < p->vec_len) {
+			if (prev->len) {
+				memcpy(&p->vec[p->vec_index], prev, sizeof(struct page_region));
+				p->vec_index++;
+			}
+			prev->start = addr;
+			prev->len = len;
+			prev->bitmap = bitmap;
+			p->found_pages += len;
+		} else {
+			return -ENOSPC;
+		}
+	}
+	return 0;
+}
+
+static inline int export_prev_to_out(struct pagemap_scan_private *p, struct page_region __user *vec,
+				     unsigned long *vec_index)
+{
+	struct page_region *prev = &p->prev;
+
+	if (prev->len) {
+		if (copy_to_user(&vec[*vec_index], prev, sizeof(struct page_region)))
+			return -EFAULT;
+		p->vec_index++;
+		(*vec_index)++;
+		prev->len = 0;
+	}
+	return 0;
+}
+
+static inline int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long addr,
+					 unsigned long end, struct mm_walk *walk)
+{
+	struct pagemap_scan_private *p = walk->private;
+	struct vm_area_struct *vma = walk->vma;
+	unsigned long start = addr;
+	unsigned int len;
+	spinlock_t *ptl;
+	int ret = 0;
+	pte_t *pte;
+	bool pmd_wt;
+
+	if ((walk->vma->vm_end < addr) || (p->max_pages && p->found_pages == p->max_pages))
+		return 0;
+	end = min(end, walk->vma->vm_end);
+
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	ptl = pmd_trans_huge_lock(pmd, vma);
+	if (ptl) {
+		pmd_wt = !is_pmd_uffd_wp(*pmd);
+		/*
+		 * Break huge page into small pages if operation needs to be performed is
+		 * on a portion of the huge page or the return buffer cannot store complete
+		 * data.
+		 */
+		if (pmd_wt && (IS_WP_ENGAGE_OP(p) && (end - addr < HPAGE_SIZE))) {
+			spin_unlock(ptl);
+			split_huge_pmd(vma, pmd, addr);
+			goto process_smaller_pages;
+		}
+		if (IS_GET_OP(p)) {
+			len = (end - addr)/PAGE_SIZE;
+			if (p->max_pages && p->found_pages + len > p->max_pages)
+				len = p->max_pages - p->found_pages;
+
+			ret = add_to_out(pmd_wt, vma->vm_file, pmd_present(*pmd),
+					 is_swap_pmd(*pmd), p, addr, len);
+			if (ret) {
+				spin_unlock(ptl);
+				return ret;
+			}
+		}
+		spin_unlock(ptl);
+		if (IS_WP_ENGAGE_OP(p) && pmd_wt) {
+			BUG_ON(start & ~HPAGE_MASK);
+
+			ret = wp_range_async(vma, addr, HPAGE_SIZE);
+		}
+		return ret;
+	}
+process_smaller_pages:
+	if (pmd_trans_unstable(pmd))
+		return 0;
+#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
+
+	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
+	for (; addr < end && !ret && (!p->max_pages || (p->found_pages < p->max_pages))
+	     ; pte++, addr += PAGE_SIZE) {
+		if (IS_GET_OP(p)) {
+			ret = add_to_out(!is_pte_uffd_wp(*pte), vma->vm_file, pte_present(*pte),
+					 is_swap_pte(*pte), p, addr, 1);
+			if (ret)
+				break;
+		}
+	}
+	pte_unmap_unlock(pte - 1, ptl);
+	if (IS_WP_ENGAGE_OP(p)) {
+		BUG_ON(start & ~PAGE_MASK);
+		ret = wp_range_async(vma, start, addr - start);
+	}
+
+	cond_resched();
+	return ret;
+}
+
+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 int len;
+	int ret = 0;
+
+	if (vma) {
+		len = (end - addr)/PAGE_SIZE;
+		if (p->max_pages && p->found_pages + len > p->max_pages)
+			len = p->max_pages - p->found_pages;
+		ret = add_to_out(false, vma->vm_file, false, false, p, addr, len);
+	}
+
+	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,
+};
+
+static long do_pagemap_cmd(struct mm_struct *mm, struct pagemap_scan_arg *arg)
+{
+	unsigned long empty_slots, vec_index = 0;
+	unsigned long __user start, end;
+	unsigned long __start, __end;
+	struct page_region __user *vec;
+	struct pagemap_scan_private p;
+	int ret;
+
+	start = (unsigned long)untagged_addr(arg->start);
+	vec = (struct page_region *)untagged_addr(arg->vec);
+	if ((!IS_ALIGNED(start, PAGE_SIZE)) || (!access_ok((void __user *)start, arg->len)))
+		return -EINVAL;
+	if (IS_GET_OP(arg) && ((arg->vec_len == 0) ||
+	    (!access_ok((void __user *)vec, arg->vec_len * sizeof(struct page_region)))))
+		return -ENOMEM;
+	if ((arg->flags & ~PAGEMAP_WP_ENGAGE) || (arg->required_mask & ~PAGEMAP_OP_MASK) ||
+	    (arg->anyof_mask & ~PAGEMAP_OP_MASK) || (arg->excluded_mask & ~PAGEMAP_OP_MASK) ||
+	    (arg->return_mask & ~PAGEMAP_OP_MASK))
+		return -EINVAL;
+	if (IS_GET_OP(arg) && ((!arg->required_mask && !arg->anyof_mask && !arg->excluded_mask) ||
+				!arg->return_mask))
+		return -EINVAL;
+	/* The non-WT flags cannot be obtained if PAGEMAP_WP_ENGAGE is also specified. */
+	if (IS_WP_ENGAGE_OP(arg) && ((arg->required_mask & PAGEMAP_NONWT_OP_MASK) ||
+	    (arg->anyof_mask & PAGEMAP_NONWT_OP_MASK)))
+		return -EINVAL;
+
+	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.prev.len = 0;
+	p.vec_len = (PAGEMAP_WALK_SIZE >> PAGE_SHIFT);
+
+	if (IS_GET_OP(arg)) {
+		p.vec = kmalloc_array(p.vec_len, sizeof(struct page_region), GFP_KERNEL);
+		if (!p.vec)
+			return -ENOMEM;
+	} else {
+		p.vec = NULL;
+	}
+
+	__start = __end = start;
+	while (__end < end) {
+		p.vec_index = 0;
+		empty_slots = arg->vec_len - vec_index;
+		if (p.vec_len > empty_slots)
+			p.vec_len = empty_slots;
+
+		__end = (__start + PAGEMAP_WALK_SIZE) & PAGEMAP_WALK_MASK;
+		if (__end > end)
+			__end = end;
+
+		mmap_read_lock(mm);
+		ret = walk_page_range(mm, __start, __end, &pagemap_scan_ops, &p);
+		mmap_read_unlock(mm);
+
+		if (!(!ret || ret == -ENOSPC))
+			goto free_data;
+
+		__start = __end;
+		if (IS_GET_OP(arg) && 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 = export_prev_to_out(&p, vec, &vec_index);
+	if (!ret)
+		ret = vec_index;
+free_data:
+	if (IS_GET_OP(arg))
+		kfree(p.vec);
+
+	return ret;
+}
+
+static long pagemap_scan_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
+{
+	struct pagemap_scan_arg __user *uarg = (struct pagemap_scan_arg __user *)arg;
+	struct mm_struct *mm = file->private_data;
+	struct pagemap_scan_arg argument;
+
+	if (cmd == PAGEMAP_SCAN) {
+		if (copy_from_user(&argument, uarg, sizeof(struct pagemap_scan_arg)))
+			return -EFAULT;
+		return do_pagemap_cmd(mm, &argument);
+	}
+	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/uapi/linux/fs.h b/include/uapi/linux/fs.h
index b7b56871029c..6d03a903a745 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -305,4 +305,54 @@ 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 pagemap_scan_arg)
+
+/* Bits are set in the bitmap of the page_region and masks in pagemap_scan_args */
+#define PAGE_IS_WT		(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
+ * bitmap:	Bits sets for the region
+ */
+struct page_region {
+	__u64 start;
+	__u64 len;
+	__u64 bitmap;
+};
+
+/*
+ * struct pagemap_scan_arg - Pagemap ioctl argument
+ * @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
+ * @flags:		Flags for the IOCTL
+ * @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 pagemap_scan_arg {
+	__u64 start;
+	__u64 len;
+	__u64 vec;
+	__u64 vec_len;
+	__u32 max_pages;
+	__u32 flags;
+	__u64 required_mask;
+	__u64 anyof_mask;
+	__u64 excluded_mask;
+	__u64 return_mask;
+};
+
+/* Special flags */
+#define PAGEMAP_WP_ENGAGE	(1 << 0)
+
 #endif /* _UAPI_LINUX_FS_H */
diff --git a/tools/include/uapi/linux/fs.h b/tools/include/uapi/linux/fs.h
index b7b56871029c..6d03a903a745 100644
--- a/tools/include/uapi/linux/fs.h
+++ b/tools/include/uapi/linux/fs.h
@@ -305,4 +305,54 @@ 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 pagemap_scan_arg)
+
+/* Bits are set in the bitmap of the page_region and masks in pagemap_scan_args */
+#define PAGE_IS_WT		(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
+ * bitmap:	Bits sets for the region
+ */
+struct page_region {
+	__u64 start;
+	__u64 len;
+	__u64 bitmap;
+};
+
+/*
+ * struct pagemap_scan_arg - Pagemap ioctl argument
+ * @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
+ * @flags:		Flags for the IOCTL
+ * @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 pagemap_scan_arg {
+	__u64 start;
+	__u64 len;
+	__u64 vec;
+	__u64 vec_len;
+	__u32 max_pages;
+	__u32 flags;
+	__u64 required_mask;
+	__u64 anyof_mask;
+	__u64 excluded_mask;
+	__u64 return_mask;
+};
+
+/* Special flags */
+#define PAGEMAP_WP_ENGAGE	(1 << 0)
+
 #endif /* _UAPI_LINUX_FS_H */
-- 
2.30.2