Re: [PATCH v8 3/4] fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about PTEs

On 1/27/23 10:36 PM, Peter Xu wrote:

On Tue, Jan 24, 2023 at 01:43:22PM +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_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_WT + 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 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.

I think you mentioned you need hugetlb support, here it says it doesn't. Which one is true?

Only THPs are supported. Hugetlbs aren't supported.

...

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

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 | 294 ++++++++++++++++++++++++++++++++++ include/uapi/linux/fs.h | 50 ++++++ tools/include/uapi/linux/fs.h | 50 ++++++ 3 files changed, 394 insertions(+)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index e35a0398db63..f9fbda00eb5c 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_any(pte)))
  

is_swap_pte() not needed, because pte_swp_uffd_wp_any() covers that.

I'll remove is_swap_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 +1780,288 @@ 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)
  

The name is confusing. It's not a mask of OPs, it's a mask of bits that are supported.

Maybe PAGEMAP_BITS_ALL?

I'll change in next revision.

I would also slightly prefer spelling WT out to be PAGE_IS_WRITTEN.

...

+#define PAGEMAP_NONWT_OP_MASK (PAGE_IS_FILE | PAGE_IS_PRESENT | PAGE_IS_SWAPPED)

Maybe PAGEMAP_BITS_WP_SUPPORTED?

I'll change it to PAGEMAP_NON_WRITTEN_BITS.

...

+#define IS_WP_ENGAGE_OP(a) (a->flags & PAGEMAP_WP_ENGAGE) +#define IS_GET_OP(a) (a->vec)

Having a->vec to imply the GET is fine, but IMHO not as clean as having each OP a bit in the flags.

How about:

#define PAGEMAP_OP_GET (1UL << 0) #define PAGEMAP_OP_WP (1UL << 1) #define PAGEMAP_OP_MASK (PAGEMAP_OP_GET | PAGEMAP_OP_WP)

?

Then a->vec requried for PAGEMAP_OP_GET.

I had something like PAGEMAP_OP_GET and PAGEMAP_OP_WP flags once until I was asked to only keep WP flag and vec != NULL will imply GET flag in a previous review. I'm ready to change again if there are more notes on it?

...

+#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))
  

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

+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) && !userfaultfd_wp(vma) && !userfaultfd_wp_async(vma))

• return -EPERM;
  

• if (IS_GET_OP(p) && HAS_NO_SPACE(p))

• return -ENOSPC;
  

As commented before, I think this can be dropped because it should never trigger.

It'll get changed in next revision.

...

• if (vma->vm_flags & VM_PFNMAP)

• return 1;
  

• return 0;

+}

+static inline int add_to_out(bool wt, bool file, bool pres, bool swap,

There're too many names that are too general to me. I suggest some better one. For this one, perhaps pagemap_scan_output()?

It'll get changed in next revision.

...

• 	     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;
  

This can be moved to below [1], we should stop scanning immediately if the condition met.

It would be definately possible. Wouldn't it be strange to return error when the operation was successful? A function should return error only when the current execution is unsuccessful. So I'm returning error only when the space is full and there is no space left in the buffer anymore.

...

• 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;
  

• }
  

[1]

Here is the only place found_pages got boosted, so checking here only should be enough, IIUC.

...

• }
• 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 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;
• unsigned int len;
• spinlock_t *ptl;
• int ret = 0;
• pte_t *pte;
• bool pmd_wt;

+#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 - start < HPAGE_SIZE))) {
  

• 	spin_unlock(ptl);
  

• 	split_huge_pmd(vma, pmd, start);
  

• 	goto process_smaller_pages;
  

• }
  

• if (IS_GET_OP(p)) {
  

• 	len = (end - start)/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, start, len);
  

• 	if (ret) {
  

• 		spin_unlock(ptl);
  

• 		return ret;
  

• 	}
  

• }
  

• spin_unlock(ptl);
  

• if (IS_WP_ENGAGE_OP(p) && pmd_wt)
  

• 	ret = wp_range_async(vma, start, 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, start, &ptl);
• if (IS_GET_OP(p)) {

• for (addr = start;
  

•      addr < end && !ret && (!p->max_pages || (p->found_pages < p->max_pages));
  

•      pte++, addr += PAGE_SIZE) {
  

• 	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 (!ret && IS_WP_ENGAGE_OP(p))

• ret = wp_range_async(vma, start, addr - start);
  

IIUC you wanted to cover the case where e.g. the output buffer is full or max_pages specified, so the partial pages still got wr-protected atomically.

In that case I think we need to at least allow ret==-ENOSPC here.

The error handling will be updated in next version.

...

• 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;

+}

+/* 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;
• 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;
  

Is ENOMEM the right error code? I think -EINVAL seems more proper.

Will be updated.

...

• 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;
  

The whole chunk is not very readable. Some emptly lines would be nice to have here and there, and some groupings. No need to keep the condition so long. How about something like:

start = (unsigned long)untagged_addr(arg->start); vec = (struct page_region *)untagged_addr(arg->vec);

    /* Detect illegal cmds */
    if ((arg->flags & ~PAGEMAP_OP_MASK))
            return -EINVAL;

    /* Validate common parameters */

if ((!IS_ALIGNED(start, PAGE_SIZE)) || (!access_ok((void __user *)start, arg->len))) return -EINVAL;

    /* Validate parameters for each cmd */

if (PAGEMAP_OP_GET(arg)) { ... }

    if (PAGEMAP_OP_WP(arg)) {
            ...
    }

I'll try to make it redable some what.

...

• 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 */

Better move the tools/ change out of the core changes, probably can be squashed into the selftest patch later.

Will do.

Thanks,

...

-- 2.30.2