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Date:   Tue, 20 Jun 2023 11:03:22 -0700
From:   Andrei Vagin <avagin@gmail.com>
To:     Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc:     Peter Xu <peterx@redhat.com>, David Hildenbrand <david@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        =?utf-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= <emmir@google.com>,
        Danylo Mocherniuk <mdanylo@google.com>,
        Paul Gofman <pgofman@codeweavers.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Mike Rapoport <rppt@kernel.org>, Nadav Amit <namit@vmware.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Shuah Khan <shuah@kernel.org>,
        Christian Brauner <brauner@kernel.org>,
        Yang Shi <shy828301@gmail.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Liam R . Howlett" <Liam.Howlett@oracle.com>,
        Yun Zhou <yun.zhou@windriver.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Alex Sierra <alex.sierra@amd.com>,
        Matthew Wilcox <willy@infradead.org>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        "Gustavo A . R . Silva" <gustavoars@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-kselftest@vger.kernel.org,
        Greg KH <gregkh@linuxfoundation.org>, kernel@collabora.com
Subject: Re: [PATCH v19 2/5] fs/proc/task_mmu: Implement IOCTL to get and
 optionally clear info about PTEs
Message-ID: <ZJHp6hSeS6lMo7qx@gmail.com>
References: <20230615141144.665148-1-usama.anjum@collabora.com>
 <20230615141144.665148-3-usama.anjum@collabora.com>
MIME-Version: 1.0
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In-Reply-To: <20230615141144.665148-3-usama.anjum@collabora.com>
Precedence: bulk

On Thu, Jun 15, 2023 at 07:11:41PM +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).
> - Find pages which have been written-to and/or write protect the pages
>   (atomic PM_SCAN_OP_GET + PM_SCAN_OP_WP)
> 
> This IOCTL can be extended to get information about more PTE bits.
> 
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
> ---
> Changes in v19:
> - Interface changes such as renaming, return mask and WP can be used
>   with any flags specified in masks
> - Internal code changes
> 
> Changes in v18:
> - Rebased on top of next-20230613
>   - ptep_get() updates
>   - remove pmd_trans_unstable() and add ACTION_AGAIN
> - Review updates (Micheal)
> 
> Changes in v17:
> - Rebased on next-20230606
> - Made make_uffd_wp_*_pte() better and minor changes
> 
> Changes in v16:
> - Fixed a corner case where kernel writes beyond user buffer by one
>   element
> - Bring back exclusive PM_SCAN_OP_WP
> - Cosmetic changes
> 
> Changes in v15:
> - Build fix:
>   - Use generic tlb flush function in pagemap_scan_pmd_entry() instead of
>     using x86 specific flush function in do_pagemap_scan()
>   - Remove #ifdef from pagemap_scan_hugetlb_entry()
>   - Use mm instead of undefined vma->vm_mm
> 
> Changes in v14:
> - Fix build error caused by #ifdef added at last minute in some configs
> 
> Changes in v13:
> - Review updates
> - mmap_read_lock_killable() instead of mmap_read_lock()
> - Replace uffd_wp_range() with helpers which increases performance
>   drastically for OP_WP operations by reducing the number of tlb
>   flushing etc
> - Add MMU_NOTIFY_PROTECTION_VMA notification for the memory range
> 
> Changes in v12:
> - Add hugetlb support to cover all memory types
> - Merge "userfaultfd: Define dummy uffd_wp_range()" with this patch
> - Review updates to the code
> 
> Changes in v11:
> - Find written pages in a better way
> - Fix a corner case (thanks Paul)
> - Improve the code/comments
> - remove ENGAGE_WP + ! GET operation
> - shorten the commit message in favour of moving documentation to
>   pagemap.rst
> 
> 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      | 526 ++++++++++++++++++++++++++++++++++++++++
>  include/linux/hugetlb.h |   1 +
>  include/uapi/linux/fs.h |  53 ++++
>  mm/hugetlb.c            |   2 +-
>  4 files changed, 581 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 507cd4e59d07..1844beea54ea 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>
> @@ -1765,11 +1766,536 @@ static int pagemap_release(struct inode *inode, struct file *file)
>  	return 0;
>  }
>  
> +#define PM_SCAN_REQUIRE_UFFD	(1ULL << 63)
> +
> +#define PM_SCAN_FOUND_MAX_PAGES	(1)
> +#define PM_SCAN_BUFFER_FULL	(-256)
> +
> +#define PM_SCAN_BITS_ALL	(PAGE_IS_WRITTEN | PAGE_IS_FILE |	\
> +				 PAGE_IS_PRESENT | PAGE_IS_SWAPPED)
> +#define PM_SCAN_OPS		(PM_SCAN_OP_GET | PM_SCAN_OP_WP)
> +#define IS_PM_SCAN_GET(flags)	(flags & PM_SCAN_OP_GET)
> +#define IS_PM_SCAN_WP(flags)	(flags & PM_SCAN_OP_WP)
> +#define PM_SCAN_FLAGS(wt, file, present, swap)	\
> +	((wt) | ((file) << 1) | ((present) << 2) | ((swap) << 3))
> +
> +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?

> +	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.

> +			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?

> +		ret = -ENOSPC;

should it be PM_SCAN_FOUND_MAX_PAGES? Otherwise, it fails the ioctl even
if it has handled some pages already.

> +		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?

> +		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.


> +		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.

> +		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
>