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

[Nadav Amit]

On 2/2/23 1:29 PM, 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.

I was not involved before, so I am not commenting on the API and code to 
avoid making unhelpful noise.

Having said that, some things in the code seem quite dirty and make 
understanding the code hard to read.

> Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
> ---
> 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))

I think that in general it is better to have an inline function instead 
of macros when possible, as it is clearer and checks types. Anyhow, IMHO 
most of these macros are better be open-coded.

> +
> +#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))
> +
> +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 (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;
The use of "len" both for bytes and pages is very confusing. Consider 
changing the name to n_pages or something similar.
> +			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 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;
I think that such goto's are really confusing and should be avoided. And 
using 'else' (could have easily prevented the need for goto). It is not 
the best solution though, since I think it would have been better to 
invert the conditions.
> +		}
> +		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);
We might have not entered the loop and pte-1 would be wrong.
> +	if ((!ret || ret == -ENOSPC) && IS_WP_ENGAGE_OP(p) && (addr - start))
What does 'addr - start' mean? If you want to say they are not equal, 
why not say so?
> +		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;

The whole point of __user (attribute) is to be assigned to pointers.

> +	unsigned long __start, __end;

I think such names do not convey sufficient information.

> +	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))

Using bitwise or to check

    (arg->required_mask | arg->anyof_mask |
     arg->excluded_mask | arg->return_mask) & ~PAGE_MAP_BITS_ALL

Would have been much cleaner, IMHO.

> +		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;
> +
> +	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;
I find it cleaner to initialize 'p.vec = NULL' unconditionally before 
IS_GET_OP() check.
> +	}
> +	__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;
Easier to understand using min().
> +
> +		mmap_read_lock(mm);
> +		ret = walk_page_range(mm, __start, __end, &pagemap_scan_ops, &p);
> +		mmap_read_unlock(mm);
> +		if (!(!ret || ret == -ENOSPC))

Double negations complicate things unnecessarily.

And if you already "break" on ret, why do you check the condition in the 
while loop?

> +			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);
Just call it unconditionally.
> +
> +	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)

These names are way too generic and are likely to be misused for the 
wrong purpose. The "_IS_" part seems confusing as well. So I think the 
naming needs to be fixed and some new type (using typedef) or enum 
should be introduced to hold these flags. I understand it is part of 
uapi and it is less common there, but it is not unheard of and does make 
things clearer.


> +
> +/*
> + * 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;

I presume in bytes. Would be useful to mention.

> +	__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 */