Date:   Mon, 20 Feb 2023 15:38:58 +0500
From:   Muhammad Usama Anjum <usama.anjum@...labora.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Peter Xu <peterx@...hat.com>,
        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>,
        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 2/17/23 3:10 PM, Mike Rapoport wrote:
> 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;
>> +}
>> +
>> +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.
Will do in next version.

> 
> And I'd also make IS_GET_OP() more explicit by defining a PAGEMAP_WP_GET or
> similar flag rather than using arg->vec.
I had in the first revisions. But explicit GET_OP was removed in the
previous iterations after some feedback. Peter has also suggested this.
I'll add the GET_OP flag again.

> 
>> +
>> +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.
Sorry, I'll add them.

> 
>> +}
>> +
>> +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;
> }
This seems a very good way. Thank you so much!

> 
>> +
>> +	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
>>
> 

-- 
BR,
Muhammad Usama Anjum

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