Date:   Wed, 8 Feb 2023 17:15:40 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michał Mirosław <emmir@...gle.com>,
        Andrei Vagin <avagin@...il.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Yun Zhou <yun.zhou@...driver.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alex Sierra <alex.sierra@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>, kernel@...labora.com
Subject: 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@...labora.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;

Sorry I should have mentioned this earlier: you can directly return here.

  return (pte_present(pte) && pte_uffd_wp(pte)) || pte_swp_uffd_wp_any(pte);

> +}
> +
> +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;

Same here.

> +}
> +
>  #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))
> +
> +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))

Should this be:

       (IS_WT_REQUIRED(p) && (!userfaultfd_wp(vma) || !userfaultfd_wp_async(vma)))

Instead?

> +		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;

Nit: switch the above two lines?

> +
> +	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 "p->found_pages + len >= p->max_pages", shouldn't this already return -ENOSPC?

> +	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++;
> +			}

IIUC you can have:

  int pagemap_scan_deposit(p)
  {
        if (p->vec_index >= p->vec_len)
                return -ENOSPC;

        if (p->prev->len) {
                memcpy(&p->vec[p->vec_index], prev, sizeof(struct page_region));
                p->vec_index++;
        }

        return 0;
  }

Then call it here.  I think it can also be called below to replace
export_prev_to_out().

> +			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;

This assignment is useless?

> +	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 think you said you'll clean this up a bit.  I don't think so..

> +
> +	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;
> +		}

I think you can move copy_to_user() to outside the loop, then call
pagemap_scan_deposit() before copy_to_user(), then I think you can drop
the ugly export_prev_to_out()..

> +	}
> +	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
> 

-- 
Peter Xu

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