Re: [PATCH v10 3/6] fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about PTEs

On Thu, Feb 02, 2023 at 04:29:12PM +0500, Muhammad Usama Anjum wrote:

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).
• 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_WRITTEN + PAGEMAP_WP_ENGAGE)

To get information about which pages have been written-to and/or write protect the pages, following must be performed first in order:

• The userfaultfd file descriptor is created with userfaultfd syscall.
• The UFFD_FEATURE_WP_ASYNC feature is set by UFFDIO_API IOCTL.
• The memory range is registered with UFFDIO_REGISTER_MODE_WP mode through UFFDIO_REGISTER IOCTL.

Then the any part of the registered memory or the whole memory region can be write protected using the UFFDIO_WRITEPROTECT IOCTL or PAGEMAP_SCAN 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 of struct page_region array 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 IOCTL doesn't support hugetlbs at the moment. No information about hugetlb can be obtained. This patch has evolved from a basic patch from Gabriel Krisman Bertazi.

Signed-off-by: Muhammad Usama Anjum usama.anjum@collabora.com

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 | 290 ++++++++++++++++++++++++++++++++++++++++ include/uapi/linux/fs.h | 50 +++++++ 2 files changed, 340 insertions(+)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index e35a0398db63..c6bde19d63d9 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)) ||

•    (pte_swp_uffd_wp_any(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 +1780,284 @@ static int pagemap_release(struct inode *inode, struct file *file) return 0; } +#define PAGEMAP_BITS_ALL (PAGE_IS_WRITTEN | PAGE_IS_FILE | \

• 			 PAGE_IS_PRESENT | PAGE_IS_SWAPPED)
  

+#define PAGEMAP_NON_WRITTEN_BITS (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 HAS_NO_SPACE(p) (p->max_pages && (p->found_pages == p->max_pages))

+#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_WRITTEN) || \

• (a->anyof_mask & PAGE_IS_WRITTEN))
  

All these macros are specific to pagemap_scan_ioctl() and should be namespaced accordingly, e.g. PM_SCAN_BITS_ALL, PM_SCAN_BITMAP etc.

Also, IS_<opname>_OP() will be more readable as PM_SCAN_OP_IS_<opname> and I'd suggest to open code IS_WP_ENGAGE_OP() and IS_GET_OP() and make HAS_NO_SPACE() and IS_WT_REQUIRED() static inlines rather than macros.

And I'd also make IS_GET_OP() more explicit by defining a PAGEMAP_WP_GET or similar flag rather than using arg->vec.

+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)

Please keep the lines under 80 characters limit.

+{

• struct pagemap_scan_private *p = walk->private;
• struct vm_area_struct *vma = walk->vma;
• if (IS_WT_REQUIRED(p) && !userfaultfd_wp(vma) && !userfaultfd_wp_async(vma))

• return -EPERM;
  

• if (vma->vm_flags & VM_PFNMAP)

• return 1;
  

• return 0;

+}

+static inline int pagemap_scan_output(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 (HAS_NO_SPACE(p))

• return -ENOSPC;
  

• if (p->max_pages && p->found_pages + len >= p->max_pages)

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

• if (!len)

• return -EINVAL;
  

• 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;

Please don't save on empty lines. Empty lines between logical pieces improve readability.

+}

+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 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;
• spinlock_t *ptl;
• int ret = 0;
• pte_t *pte;

+#ifdef CONFIG_TRANSPARENT_HUGEPAGE

• ptl = pmd_trans_huge_lock(pmd, vma);
• if (ptl) {

• bool pmd_wt;
  

• 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.
  

•  */
  

• if (pmd_wt && IS_WP_ENGAGE_OP(p) && (end - start < HPAGE_SIZE)) {
  

• 	spin_unlock(ptl);
  

• 	split_huge_pmd(vma, pmd, start);
  

• 	goto process_smaller_pages;
  

• }
  

• if (IS_GET_OP(p))
  

• 	ret = pagemap_scan_output(pmd_wt, vma->vm_file, pmd_present(*pmd),
  

• 				  is_swap_pmd(*pmd), p, start,
  

• 				  (end - start)/PAGE_SIZE);
  

• spin_unlock(ptl);
  

• if (!ret) {
  

• 	if (pmd_wt && IS_WP_ENGAGE_OP(p))
  

• 		uffd_wp_range(walk->mm, vma, start, HPAGE_SIZE, true);
  

• }
  

• 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, start, &ptl);
• if (IS_GET_OP(p)) {

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

• 	ret = pagemap_scan_output(!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 ((!ret || ret == -ENOSPC) && IS_WP_ENGAGE_OP(p) && (addr - start))

• uffd_wp_range(walk->mm, vma, start, addr - start, true);
  

• 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;
• int ret = 0;
• if (vma)

• ret = pagemap_scan_output(false, vma->vm_file, false, false, p, addr,
  

• 			  (end - addr)/PAGE_SIZE);
  

• return ret;

+}

+/* No hugetlb support is present. */ +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 = 0;
• start = (unsigned long)untagged_addr(arg->start);
• vec = (struct page_region *)(unsigned long)untagged_addr(arg->vec);
• /* Validate memory ranges */
• 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 -EINVAL;
  

• /* Detect illegal flags and masks */
• if ((arg->flags & ~PAGEMAP_WP_ENGAGE) || (arg->required_mask & ~PAGEMAP_BITS_ALL) ||

•    (arg->anyof_mask & ~PAGEMAP_BITS_ALL) || (arg->excluded_mask & ~PAGEMAP_BITS_ALL) ||
  

•    (arg->return_mask & ~PAGEMAP_BITS_ALL))
  

• 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_NON_WRITTEN_BITS) ||

•    (arg->anyof_mask & PAGEMAP_NON_WRITTEN_BITS)))
  

• return -EINVAL;
  

I'd split argument validation into a separate function and split the OR'ed conditions into separate if statements, e.g

bool pm_scan_args_valid(struct pagemap_scan_arg *arg) { if (IS_GET_OP(arg)) { if (!arg->return_mask) return false; if (!arg->required_mask && !arg->anyof_mask && !arg->excluded_mask) return false; }

/* ... */

return true; }

• 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 (!ret && __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..1ae9a8684b48 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_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
• • 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