Re: [PATCH v19 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

[Muhammad Usama Anjum]

Thanks for reply.

On 6/22/23 12:45 AM, Andrei Vagin wrote:
> On Wed, Jun 21, 2023 at 11:34:54AM +0500, Muhammad Usama Anjum wrote:
>> Hi,
>>
>> Thank you for your review.
>>
>> On 6/20/23 11:03 PM, Andrei Vagin wrote:
>> ...
>>>> +struct pagemap_scan_private {
>>>> +	struct page_region *vec_buf, cur_buf;
>>>> +	unsigned long long vec_buf_len, vec_buf_index, max_pages, found_pages, flags;
>>>
>>> should it be just unsigned long?
>> These internal values are storing data coming from user in struct
>> pm_scan_arg in which all variables are 64 bit(__u64) explicitly. This is
>> why we have unsigned long long here. It is absolutely necessary.
> 
> It isn't necessary, because "unsigned long" is enough for  all valid values
I guess I can make some variables here long only.

>>
>>>
>>>> +	unsigned long long required_mask, anyof_mask, excluded_mask, return_mask;
>>>> +};
>>>> +
>>>> +static inline bool is_pte_uffd_wp(pte_t pte)
>>>> +{
>>>> +	return (pte_present(pte) && pte_uffd_wp(pte)) ||
>>>> +	       pte_swp_uffd_wp_any(pte);
>>>> +}
>>>> +
>>>> +static inline void make_uffd_wp_pte(struct vm_area_struct *vma,
>>>> +				    unsigned long addr, pte_t *pte)
>>>> +{
>>>> +	pte_t ptent = ptep_get(pte);
>>>> +
>>>> +	if (pte_present(ptent)) {
>>>> +		pte_t old_pte;
>>>> +
>>>> +		old_pte = ptep_modify_prot_start(vma, addr, pte);
>>>> +		ptent = pte_mkuffd_wp(ptent);
>>>> +		ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent);
>>>> +	} else if (is_swap_pte(ptent)) {
>>>> +		ptent = pte_swp_mkuffd_wp(ptent);
>>>> +		set_pte_at(vma->vm_mm, addr, pte, ptent);
>>>> +	} else {
>>>> +		set_pte_at(vma->vm_mm, addr, pte,
>>>> +			   make_pte_marker(PTE_MARKER_UFFD_WP));
>>>> +	}
>>>> +}
>>>> +
>>>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>>> +static inline bool is_pmd_uffd_wp(pmd_t pmd)
>>>> +{
>>>> +	return (pmd_present(pmd) && pmd_uffd_wp(pmd)) ||
>>>> +	       (is_swap_pmd(pmd) && pmd_swp_uffd_wp(pmd));
>>>> +}
>>>> +
>>>> +static inline void make_uffd_wp_pmd(struct vm_area_struct *vma,
>>>> +				    unsigned long addr, pmd_t *pmdp)
>>>> +{
>>>> +	pmd_t old, pmd = *pmdp;
>>>> +
>>>> +	if (pmd_present(pmd)) {
>>>> +		old = pmdp_invalidate_ad(vma, addr, pmdp);
>>>> +		pmd = pmd_mkuffd_wp(old);
>>>> +		set_pmd_at(vma->vm_mm, addr, pmdp, pmd);
>>>> +	} else if (is_migration_entry(pmd_to_swp_entry(pmd))) {
>>>> +		pmd = pmd_swp_mkuffd_wp(pmd);
>>>> +		set_pmd_at(vma->vm_mm, addr, pmdp, pmd);
>>>> +	}
>>>> +}
>>>> +#endif
>>>> +
>>>> +#ifdef CONFIG_HUGETLB_PAGE
>>>> +static inline bool is_huge_pte_uffd_wp(pte_t pte)
>>>> +{
>>>> +	return (pte_present(pte) && huge_pte_uffd_wp(pte)) ||
>>>> +	       pte_swp_uffd_wp_any(pte);
>>>> +}
>>>> +
>>>> +static inline void make_uffd_wp_huge_pte(struct vm_area_struct *vma,
>>>> +					 unsigned long addr, pte_t *ptep,
>>>> +					 pte_t ptent)
>>>> +{
>>>> +	if (is_hugetlb_entry_hwpoisoned(ptent) || is_pte_marker(ptent))
>>>> +		return;
>>>> +
>>>> +	if (is_hugetlb_entry_migration(ptent))
>>>> +		set_huge_pte_at(vma->vm_mm, addr, ptep,
>>>> +				pte_swp_mkuffd_wp(ptent));
>>>> +	else if (!huge_pte_none(ptent))
>>>> +		huge_ptep_modify_prot_commit(vma, addr, ptep, ptent,
>>>> +					     huge_pte_mkuffd_wp(ptent));
>>>> +	else
>>>> +		set_huge_pte_at(vma->vm_mm, addr, ptep,
>>>> +				make_pte_marker(PTE_MARKER_UFFD_WP));
>>>> +}
>>>> +#endif
>>>> +
>>>> +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 ((p->flags & PM_SCAN_REQUIRE_UFFD) && (!userfaultfd_wp_async(vma) ||
>>>> +	    !userfaultfd_wp_use_markers(vma)))
>>>> +		return -EPERM;
>>>> +
>>>> +	if (vma->vm_flags & VM_PFNMAP)
>>>> +		return 1;
>>>> +
>>>> +	return 0;
>>>> +}
>>>> +
>>>> +static int pagemap_scan_output(unsigned long bitmap,
>>>> +			       struct pagemap_scan_private *p,
>>>> +			       unsigned long addr, unsigned int n_pages)
>>>> +{
>>>> +	struct page_region *cur_buf = &p->cur_buf;
>>>> +
>>>> +	if (!n_pages)
>>>> +		return -EINVAL;
>>>> +
>>>> +	bitmap &= p->return_mask;
>>>> +
>>>> +	if (cur_buf->flags == bitmap &&
>>>> +	    cur_buf->start + cur_buf->len * PAGE_SIZE == addr) {
>>>> +		cur_buf->len += n_pages;
>>>> +		p->found_pages += n_pages;
>>>> +	} else {
>>>> +		if (cur_buf->len) {
>>>
>>> I would add a comment that vec_buf_len has been decremented by one for
>>> cur_buf.
>>>
>>>> +			if (p->vec_buf_index >= p->vec_buf_len)
>>>> +				return PM_SCAN_BUFFER_FULL;
>>>> +
>>>> +			memcpy(&p->vec_buf[p->vec_buf_index], cur_buf,
>>>> +			       sizeof(*p->vec_buf));
>>>> +			p->vec_buf_index++;
>>>> +		}
>>>> +
>>>> +		cur_buf->start = addr;
>>>> +		cur_buf->len = n_pages;
>>>> +		cur_buf->flags = bitmap;
>>>> +		p->found_pages += n_pages;
>>>> +	}
>>>> +
>>>> +	if (p->found_pages == p->max_pages)
>>>> +		return PM_SCAN_FOUND_MAX_PAGES;
>>>> +
>>>> +	return 0;
>>>> +}
>>>> +
>>>> +static bool pagemap_scan_is_interesting_page(unsigned long bitmap,
>>>> +					     struct pagemap_scan_private *p)
>>>> +{
>>>> +	if ((p->required_mask & bitmap) != p->required_mask)
>>>> +		return false;
>>>> +	if (p->anyof_mask && !(p->anyof_mask & bitmap))
>>>> +		return false;
>>>> +	if (p->excluded_mask & bitmap)
>>>> +		return false;
>>>> +
>>>> +	return true;
>>>> +}
>>>> +
>>>> +static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
>>>> +				  unsigned long end, struct mm_walk *walk)
>>>> +{
>>>> +	bool is_written, flush = false, is_interesting = true;
>>>> +	struct pagemap_scan_private *p = walk->private;
>>>> +	struct vm_area_struct *vma = walk->vma;
>>>> +	unsigned long bitmap, addr = end;
>>>> +	pte_t *pte, *orig_pte, ptent;
>>>> +	spinlock_t *ptl;
>>>> +	int ret = 0;
>>>> +
>>>> +	arch_enter_lazy_mmu_mode();
>>>> +
>>>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>>> +	ptl = pmd_trans_huge_lock(pmd, vma);
>>>> +	if (ptl) {
>>>> +		unsigned long n_pages = (end - start)/PAGE_SIZE;
>>>> +
>>>> +		if (p->max_pages && n_pages > p->max_pages - p->found_pages)
>>>> +			n_pages = p->max_pages - p->found_pages;
>>>> +
>>>> +		is_written = !is_pmd_uffd_wp(*pmd);
>>>> +
>>>> +		/*
>>>> +		 * Break huge page into small pages if the WP operation need to
>>>> +		 * be performed is on a portion of the huge page.
>>>> +		 */
>>>> +		if (is_written && IS_PM_SCAN_WP(p->flags) &&
>>>> +		    n_pages < HPAGE_SIZE/PAGE_SIZE) {
>>>
>>> It might be worth stopping rather than splitting the huge page.
>> Sometimes normal pages can be grouped to become THP by the kernel depending
>> upon the configurations. We don't want to reject ioctl just because this
>> folding operation happened. We don't want to stop rather spliting is the
>> best option and it does work fine.
>>
>>>
>>>> +			spin_unlock(ptl);
>>>> +
>>>> +			split_huge_pmd(vma, pmd, start);
>>>> +			goto process_smaller_pages;
>>>> +		}
>>>> +
>>>> +		bitmap = PM_SCAN_FLAGS(is_written, (bool)vma->vm_file,
>>>> +				       pmd_present(*pmd), is_swap_pmd(*pmd));
>>>> +
>>>> +		if (IS_PM_SCAN_GET(p->flags)) {
>>>> +			is_interesting = pagemap_scan_is_interesting_page(bitmap, p);
>>>> +			if (is_interesting)
>>>> +				ret = pagemap_scan_output(bitmap, p, start, n_pages);
>>>> +		}
>>>> +
>>>> +		if (IS_PM_SCAN_WP(p->flags) && is_written && is_interesting &&
>>>> +		    ret >= 0) {
>>>> +			make_uffd_wp_pmd(vma, start, pmd);
>>>> +			flush_tlb_range(vma, start, end);
>>>> +		}
>>>> +
>>>> +		spin_unlock(ptl);
>>>> +
>>>> +		arch_leave_lazy_mmu_mode();
>>>> +		return ret;
>>>> +	}
>>>> +
>>>> +process_smaller_pages:
>>>> +#endif
>>>> +
>>>> +	orig_pte = pte = pte_offset_map_lock(vma->vm_mm, pmd, start, &ptl);
>>>> +	if (!pte) {
>>>> +		walk->action = ACTION_AGAIN;
>>>> +		return 0;
>>>> +	}
>>>> +
>>>> +	for (addr = start; addr < end && !ret; pte++, addr += PAGE_SIZE) {
>>>> +		ptent = ptep_get(pte);
>>>> +		is_written = !is_pte_uffd_wp(ptent);
>>>> +
>>>> +		bitmap = PM_SCAN_FLAGS(is_written, (bool)vma->vm_file,
>>>> +				       pte_present(ptent), is_swap_pte(ptent));
>>>> +
>>>> +		if (IS_PM_SCAN_GET(p->flags)) {
>>>> +			is_interesting = pagemap_scan_is_interesting_page(bitmap, p);
>>>> +			if (is_interesting)
>>>> +				ret = pagemap_scan_output(bitmap, p, addr, 1);
>>>> +		}
>>>> +
>>>> +		if (IS_PM_SCAN_WP(p->flags) && is_written && is_interesting &&
>>>> +		    ret >= 0) {
>>>> +			make_uffd_wp_pte(vma, addr, pte);
>>>> +			flush = true;
>>>> +		}
>>>> +	}
>>>> +
>>>> +	if (flush)
>>>> +		flush_tlb_range(vma, start, addr);
>>>> +
>>>> +	pte_unmap_unlock(orig_pte, ptl);
>>>> +	arch_leave_lazy_mmu_mode();
>>>> +
>>>> +	cond_resched();
>>>> +	return ret;
>>>> +}
>>>> +
>>>> +#ifdef CONFIG_HUGETLB_PAGE
>>>> +static int pagemap_scan_hugetlb_entry(pte_t *ptep, unsigned long hmask,
>>>> +				      unsigned long start, unsigned long end,
>>>> +				      struct mm_walk *walk)
>>>> +{
>>>> +	unsigned long n_pages = (end - start)/PAGE_SIZE;
>>>> +	struct pagemap_scan_private *p = walk->private;
>>>> +	struct vm_area_struct *vma = walk->vma;
>>>> +	bool is_written, is_interesting = true;
>>>> +	struct hstate *h = hstate_vma(vma);
>>>> +	unsigned long bitmap;
>>>> +	spinlock_t *ptl;
>>>> +	int ret = 0;
>>>> +	pte_t ptent;
>>>> +
>>>> +	if (IS_PM_SCAN_WP(p->flags) && n_pages < HPAGE_SIZE/PAGE_SIZE)
>>>> +		return -EINVAL;
>>>> +
>>>> +	if (p->max_pages && n_pages > p->max_pages - p->found_pages)
>>>> +		n_pages = p->max_pages - p->found_pages;
>>>> +
>>>> +	if (IS_PM_SCAN_WP(p->flags)) {
>>>> +		i_mmap_lock_write(vma->vm_file->f_mapping);
>>>> +		ptl = huge_pte_lock(h, vma->vm_mm, ptep);
>>>> +	}
>>>> +
>>>> +	ptent = huge_ptep_get(ptep);
>>>> +	is_written = !is_huge_pte_uffd_wp(ptent);
>>>> +
>>>> +	/*
>>>> +	 * Partial hugetlb page clear isn't supported
>>>> +	 */
>>>> +	if (is_written && IS_PM_SCAN_WP(p->flags) &&
>>>> +	    n_pages < HPAGE_SIZE/PAGE_SIZE) {
>>>
>>> n_pages depends on found_pages that is incremented in
>>> pagemap_scan_output. Should we do this check right before calling
>>> pagemap_scan_output?
>> This condition is acting like a check before pagemap_scan_output() already.
>> n_pages is telling us that how many empty pages are left to find out. But
>> if by making n_pages less, we cannot cover the whole hugetlb, we should
>> just return error as we don't split the hugetlb.
> 
> I understand that, but here you don't know whether pagemap_scan_output
> will be called for this page or not. It means that an error can be
> reported due to unrelated page and this doesn't look right.
> 
>>
>>>
>>>> +		ret = -ENOSPC;
>>>
>>> should it be PM_SCAN_FOUND_MAX_PAGES? Otherwise, it fails the ioctl even
>>> if it has handled some pages already.
>> It is a double edge sword. If we don't return error, user will never know
>> that he needs to specify more max_pages or his output buffer is small and
>> not coverig the entire range. The real purpose here that user gets aware
>> that he needs to specify full hugetlb range and found pages should cover
>> the entire range as well.
> 
> but if PM_SCAN_OP_WP is set, this error will be fatal, because some
> pages can be marked write-protected, but they are not be reported to
> user-space. 
> 
> I think the ioctl has to report back the end address of the handled
> region. It is like read and write syscalls that can return less data
> than requested.
This is good point. I'll abort the walk here instead of retuning the error
to user.

> 
>>
>>>
>>>> +		goto unlock_and_return;
>>>> +	}
>>>> +
>>>> +	bitmap = PM_SCAN_FLAGS(is_written, (bool)vma->vm_file,
>>>> +			       pte_present(ptent), is_swap_pte(ptent));
>>>> +
>>>> +	if (IS_PM_SCAN_GET(p->flags)) {
>>>> +		is_interesting = pagemap_scan_is_interesting_page(bitmap, p);
>>>> +		if (is_interesting)
>>>> +			ret = pagemap_scan_output(bitmap, p, start, n_pages);
>>>> +	}
>>>> +
>>>> +	if (IS_PM_SCAN_WP(p->flags) && is_written && is_interesting &&
>>>> +	    ret >= 0) {
>>>> +		make_uffd_wp_huge_pte(vma, start, ptep, ptent);
>>>> +		flush_hugetlb_tlb_range(vma, start, end);
>>>> +	}
>>>> +
>>>> +unlock_and_return:
>>>> +	if (IS_PM_SCAN_WP(p->flags)) {
>>>> +		spin_unlock(ptl);
>>>> +		i_mmap_unlock_write(vma->vm_file->f_mapping);
>>>> +	}
>>>> +
>>>> +	return ret;
>>>> +}
>>>> +#else
>>>> +#define pagemap_scan_hugetlb_entry NULL
>>>> +#endif
>>>> +
>>>> +static int pagemap_scan_pte_hole(unsigned long addr, unsigned long end,
>>>> +				 int depth, struct mm_walk *walk)
>>>> +{
>>>> +	unsigned long n_pages = (end - addr)/PAGE_SIZE;
>>>> +	struct pagemap_scan_private *p = walk->private;
>>>> +	struct vm_area_struct *vma = walk->vma;
>>>> +	int ret = 0;
>>>> +
>>>> +	if (!vma || !IS_PM_SCAN_GET(p->flags))
>>>> +		return 0;
>>>> +
>>>> +	if (p->max_pages && n_pages > p->max_pages - p->found_pages)
>>>> +		n_pages = p->max_pages - p->found_pages;
>>>> +
>>>> +	ret = pagemap_scan_output(PM_SCAN_FLAGS(false, (bool)vma->vm_file,
>>>> +				  false, false), p, addr, n_pages);
>>>> +
>>>> +	return ret;
>>>> +}
>>>> +
>>>> +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,
>>>> +	.hugetlb_entry = pagemap_scan_hugetlb_entry,
>>>> +};
>>>> +
>>>> +static int pagemap_scan_args_valid(struct pm_scan_arg *arg, unsigned long start,
>>>> +				   struct page_region __user *vec)
>>>> +{
>>>> +	/* Detect illegal size, flags, len and masks */
>>>> +	if (arg->size != sizeof(struct pm_scan_arg))
>>>> +		return -EINVAL;
>>>> +	if (!arg->flags)
>>>> +		return -EINVAL;
>>>> +	if (arg->flags & ~PM_SCAN_OPS)
>>>> +		return -EINVAL;
>>>> +	if (!arg->len)
>>>> +		return -EINVAL;
>>>> +	if ((arg->required_mask | arg->anyof_mask | arg->excluded_mask |
>>>> +	     arg->return_mask) & ~PM_SCAN_BITS_ALL)
>>>> +		return -EINVAL;
>>>> +	if (!arg->required_mask && !arg->anyof_mask &&
>>>> +	    !arg->excluded_mask)
>>>> +		return -EINVAL;
>>>> +	if (!arg->return_mask)
>>>> +		return -EINVAL;
>>>> +
>>>> +	/* Validate memory range */
>>>> +	if (!IS_ALIGNED(start, PAGE_SIZE))
>>>> +		return -EINVAL;
>>>> +	if (!access_ok((void __user *)start, arg->len))
>>>> +		return -EFAULT;
>>>> +
>>>> +	if (IS_PM_SCAN_GET(arg->flags)) {
>>>> +		if (!arg->vec)
>>>> +			return -EINVAL;
>>>> +		if (arg->vec_len == 0)
>>>> +			return -EINVAL;
>>>> +		if (!access_ok((void __user *)arg->vec,
>>>> +			       arg->vec_len * sizeof(struct page_region)))
>>>> +			return -EFAULT;
>>>> +	}
>>>> +
>>>> +	if (IS_PM_SCAN_WP(arg->flags) && !IS_PM_SCAN_GET(arg->flags) &&
>>>> +	    arg->max_pages)
>>>> +		return -EINVAL;
>>>> +
>>>> +	return 0;
>>>> +}
>>>> +
>>>> +static long do_pagemap_scan(struct mm_struct *mm, unsigned long __arg)
>>>> +{
>>>> +	struct pm_scan_arg __user *uarg = (struct pm_scan_arg __user *)__arg;
>>>> +	unsigned long long start, end, walk_start, walk_end;
>>>> +	unsigned long long empty_slots, vec_index = 0;
>>>> +	struct mmu_notifier_range range;
>>>> +	struct page_region __user *vec;
>>>> +	struct pagemap_scan_private p;
>>>> +	struct pm_scan_arg arg;
>>>> +	int ret = 0;
>>>> +
>>>> +	if (copy_from_user(&arg, uarg, sizeof(arg)))
>>>> +		return -EFAULT;
>>>> +
>>>> +	start = untagged_addr((unsigned long)arg.start);
>>>> +	vec = (struct page_region *)untagged_addr((unsigned long)arg.vec);
>>>> +
>>>> +	ret = pagemap_scan_args_valid(&arg, start, vec);
>>>> +	if (ret)
>>>> +		return ret;
>>>> +
>>>> +	end = start + arg.len;
>>>> +	p.max_pages = arg.max_pages;
>>>> +	p.found_pages = 0;
>>>> +	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.flags = arg.flags;
>>>> +	p.flags |= ((p.required_mask | p.anyof_mask | p.excluded_mask) &
>>>> +		    PAGE_IS_WRITTEN) ? PM_SCAN_REQUIRE_UFFD : 0;
>>>> +	p.cur_buf.start = p.cur_buf.len = p.cur_buf.flags = 0;
>>>> +	p.vec_buf = NULL;
>>>> +	p.vec_buf_len = PAGEMAP_WALK_SIZE >> PAGE_SHIFT;
>>>> +
>>>> +	/*
>>>> +	 * Allocate smaller buffer to get output from inside the page walk
>>>> +	 * functions and walk page range in PAGEMAP_WALK_SIZE size chunks. As
>>>> +	 * we want to return output to user in compact form where no two
>>>> +	 * consecutive regions should be continuous and have the same flags.
>>>> +	 * So store the latest element in p.cur_buf between different walks and
>>>> +	 * store the p.cur_buf at the end of the walk to the user buffer.
>>>> +	 */
>>>> +	if (IS_PM_SCAN_GET(p.flags)) {
>>>> +		p.vec_buf = kmalloc_array(p.vec_buf_len, sizeof(*p.vec_buf),
>>>> +					  GFP_KERNEL);
>>>> +		if (!p.vec_buf)
>>>> +			return -ENOMEM;
>>>> +	}
>>>> +
>>>> +	if (IS_PM_SCAN_WP(p.flags)) {
>>>> +		mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_VMA, 0,
>>>> +					mm, start, end);
>>>> +		mmu_notifier_invalidate_range_start(&range);
>>>> +	}
>>>> +
>>>> +	walk_start = walk_end = start;
>>>> +	while (walk_end < end && !ret) {
>>>
>>> How long can it take to run this loop? Should we interrupt it if a
>>> signal has been queued?
>> I'm following mincore and pagemap_read here. There is no such thing there.
>> IOCTL is performing what user has requested.
> 
> In case of pagemap, its buffer is limited by MAX_RW_COUNT (0x7ffff000),
> so it can handle maximum 0xffffe00 pages in one iteration.  Do you have any
> limits for input parameters?
> 
>> If the execution time is a
>> concern, user should have called the IOCTL on shorter address range.
> 
> It doesn't work this way. There can be many users and signals has to be
> delivered in a reasonable time. One of the obvious examples when a signal
> has to be delivered asap is OOM.
This IOCTL is just like mincore, but with other flags and functionalities.
Mincore doesn't put any limit like this. I don't think we should put any
limit here as well.

> 
>>
>>>
>>>> +		if (IS_PM_SCAN_GET(p.flags)) {
>>>> +			p.vec_buf_index = 0;
>>>> +
>>>> +			/*
>>>> +			 * All data is copied to cur_buf first. When more data
>>>> +			 * is found, we push cur_buf to vec_buf and copy new
>>>> +			 * data to cur_buf. Subtract 1 from length as the
>>>> +			 * index of cur_buf isn't counted in length.
>>>> +			 */
>>>> +			empty_slots = arg.vec_len - vec_index;
>>>> +			p.vec_buf_len = min(p.vec_buf_len, empty_slots - 1);
>>>> +		}
>>>> +
>>>> +		walk_end = (walk_start + PAGEMAP_WALK_SIZE) & PAGEMAP_WALK_MASK;
>>>> +		if (walk_end > end)
>>>> +			walk_end = end;
>>>> +
>>>> +		ret = mmap_read_lock_killable(mm);
>>>> +		if (ret)
>>>> +			goto free_data;
>>>
>>> This will fail the ioctl, but it isn't acceptable if any pages has been
>>> handled.
>> I'm following pagemap_read() here.
> 
> 
> pagemap_read() doesn't change states of pages, so it is safe to return
> errors from it. In this case, such errors will be fatal, because you
> mark some pages as write-protected but don't report them to the user.
Should we change to mmap_read_lock() instead or abort gracefully?

> 
>>
>>>
>>>
>>>> +		ret = walk_page_range(mm, walk_start, walk_end,
>>>> +				      &pagemap_scan_ops, &p);
>>>> +		mmap_read_unlock(mm);
>>>> +
>>>
>>> ret can be ENOSPC returned from pagemap_scan_pmd_entry, but we should
>>> not return this error if any pages has been handled.
>> ENOSPC isn't being returned from pagemap_scan_pmd_entry now.
> 
> I mean pagemap_scan_hugetlb_entry.
> 
>>
>>>
>>>> +		if (ret && ret != PM_SCAN_BUFFER_FULL &&
>>>> +		    ret != PM_SCAN_FOUND_MAX_PAGES)
>>>> +			goto free_data;
>>>> +
>>>> +		walk_start = walk_end;
>>>> +		if (IS_PM_SCAN_GET(p.flags) && p.vec_buf_index) {
>>>> +			if (copy_to_user(&vec[vec_index], p.vec_buf,
>>>> +					 p.vec_buf_index * sizeof(*p.vec_buf))) {
>>>> +				/*
>>>> +				 * Return error even though the OP succeeded
>>>> +				 */
>>>> +				ret = -EFAULT;
>>>> +				goto free_data;
>>>> +			}
>>>> +			vec_index += p.vec_buf_index;
>>>> +		}
>>>> +	}
>>>> +
>>>> +	if (p.cur_buf.len) {
>>>> +		if (copy_to_user(&vec[vec_index], &p.cur_buf, sizeof(p.cur_buf))) {
>>>> +			ret = -EFAULT;
>>>> +			goto free_data;
>>>> +		}
>>>> +		vec_index++;
>>>> +	}
>>>> +
>>>> +	ret = vec_index;
>>>> +
>>>> +free_data:
>>>> +	if (IS_PM_SCAN_WP(p.flags))
>>>> +		mmu_notifier_invalidate_range_end(&range);
>>>> +
>>>> +	kfree(p.vec_buf);
>>>> +	return ret;
>>>> +}
>>>> +
>>>> +static long do_pagemap_cmd(struct file *file, unsigned int cmd,
>>>> +			   unsigned long arg)
>>>> +{
>>>> +	struct mm_struct *mm = file->private_data;
>>>> +
>>>> +	switch (cmd) {
>>>> +	case PAGEMAP_SCAN:
>>>> +		return do_pagemap_scan(mm, arg);
>>>> +
>>>> +	default:
>>>> +		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 = do_pagemap_cmd,
>>>> +	.compat_ioctl	= do_pagemap_cmd,
>>>>  };
>>>>  #endif /* CONFIG_PROC_PAGE_MONITOR */
>>>>  
>>>> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
>>>> index beb7c63d2871..a6e773c3e2b4 100644
>>>> --- a/include/linux/hugetlb.h
>>>> +++ b/include/linux/hugetlb.h
>>>> @@ -261,6 +261,7 @@ long hugetlb_change_protection(struct vm_area_struct *vma,
>>>>  		unsigned long cp_flags);
>>>>  
>>>>  bool is_hugetlb_entry_migration(pte_t pte);
>>>> +bool is_hugetlb_entry_hwpoisoned(pte_t pte);
>>>>  void hugetlb_unshare_all_pmds(struct vm_area_struct *vma);
>>>>  
>>>>  #else /* !CONFIG_HUGETLB_PAGE */
>>>> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
>>>> index b7b56871029c..a9fb44db84a3 100644
>>>> --- a/include/uapi/linux/fs.h
>>>> +++ b/include/uapi/linux/fs.h
>>>> @@ -305,4 +305,57 @@ 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 pm_scan_arg)
>>>> +
>>>> +/* Bits are set in flags of the page_region and masks in pm_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 flags
>>>> + * @start:	Start of the region
>>>> + * @len:	Length of the region in pages
>>>> + * @bitmap:	Bits sets for the region
>>>> + */
>>>> +struct page_region {
>>>> +	__u64 start;
>>>> +	__u64 len;
>>>> +	__u64 flags;
>>>> +};
>>>> +
>>>> +/*
>>>> + * struct pm_scan_arg - Pagemap ioctl argument
>>>> + * @size:		Size of the structure
>>>> + * @flags:		Flags for the IOCTL
>>>> + * @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
>>>> + * @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 pm_scan_arg {
>>>> +	__u64 size;
>>>> +	__u64 flags;
>>>> +	__u64 start;
>>>> +	__u64 len;
>>>> +	__u64 vec;
>>>> +	__u64 vec_len;
>>>> +	__u64 max_pages;
>>>> +	__u64 required_mask;
>>>> +	__u64 anyof_mask;
>>>> +	__u64 excluded_mask;
>>>> +	__u64 return_mask;
>>>> +};
>>>> +
>>>> +/* Supported flags */
>>>> +#define PM_SCAN_OP_GET	(1 << 0)
>>>> +#define PM_SCAN_OP_WP	(1 << 1)
>>>> +
>>>>  #endif /* _UAPI_LINUX_FS_H */
>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>>> index 0db13167b1ee..7e60f0f3fd03 100644
>>>> --- a/mm/hugetlb.c
>>>> +++ b/mm/hugetlb.c
>>>> @@ -4980,7 +4980,7 @@ bool is_hugetlb_entry_migration(pte_t pte)
>>>>  		return false;
>>>>  }
>>>>  
>>>> -static bool is_hugetlb_entry_hwpoisoned(pte_t pte)
>>>> +bool is_hugetlb_entry_hwpoisoned(pte_t pte)
>>>>  {
>>>>  	swp_entry_t swp;
>>>>  
>>>> -- 
>>>> 2.39.2
>>>>
>>
>> -- 
>> BR,
>> Muhammad Usama Anjum

-- 
BR,
Muhammad Usama Anjum