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Date:   Fri, 21 Jul 2023 15:48:22 +0500
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Cc:     Muhammad Usama Anjum <usama.anjum@collabora.com>,
        Alex Sierra <alex.sierra@amd.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Christian Brauner <brauner@kernel.org>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dan Williams <dan.j.williams@intel.com>,
        David Hildenbrand <david@redhat.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        "Gustavo A . R . Silva" <gustavoars@kernel.org>,
        "Liam R . Howlett" <Liam.Howlett@Oracle.com>,
        Matthew Wilcox <willy@infradead.org>,
        =?UTF-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= <emmir@google.com>,
        Mike Rapoport <rppt@kernel.org>, Nadav Amit <namit@vmware.com>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Paul Gofman <pgofman@codeweavers.com>,
        Peter Xu <peterx@redhat.com>, Shuah Khan <shuah@kernel.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Yang Shi <shy828301@gmail.com>,
        Yun Zhou <yun.zhou@windriver.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-kselftest@vger.kernel.org,
        kernel@collabora.com
Subject: Re: fs/proc/task_mmu: Implement IOCTL for efficient page table
 scanning
Content-Language: en-US
To:     =?UTF-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= <mirq-linux@rere.qmqm.pl>,
        Andrei Vagin <avagin@gmail.com>,
        Danylo Mocherniuk <mdanylo@google.com>
References: <20230713101415.108875-6-usama.anjum@collabora.com>
 <a0b5c6776b2ed91f78a7575649f8b100e58bd3a9.1689881078.git.mirq-linux@rere.qmqm.pl>
From:   Muhammad Usama Anjum <usama.anjum@collabora.com>
In-Reply-To: <a0b5c6776b2ed91f78a7575649f8b100e58bd3a9.1689881078.git.mirq-linux@rere.qmqm.pl>
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On 7/21/23 12:28 AM, Michał Mirosław wrote:
> This is a massaged version of patch by Muhammad Usama Anjum [1]
> to illustrate my review comments and hopefully push the implementation
> efforts closer to conclusion. The changes are:
Thank you so much for this effort. I also want to reach conclusion. I'll
agree with all the changes which don't affect me. But some requirements
aren't being fulfilled with this current design.

> 
> 1. the API:
> 
>   a. return ranges as {begin, end} instead of {begin + len};
Agreed.

>   b. rename match "flags" to 'page categories' everywhere - this makes
> 	it easier to differentiate the ioctl()s categorisation of pages
> 	from struct page flags;
I've no problem with it.

#define PAGE_IS_WPASYNC		(1 << 0)
#define PAGE_IS_WRITTEN		(1 << 1)
You have another new flag PAGE_IS_WPASYNC. But there is no application of
PAGE_IS_WPASYNC. We must not add a flag which don't have any user.

>   c. change {required + excluded} to {inverted + required}. This was
> 	rejected before, but I'd like to illustrate the difference.
> 	Old interface can be translated to the new by:
> 		categories_inverted = excluded_mask
> 		categories_mask = required_mask | excluded_mask
> 		categories_anyof_mask = anyof_mask
> 	The new way allows filtering by: A & (B | !C)
> 		categories_inverted = C
> 		categories_mask = A
> 		categories_anyof_mask = B | C
I'm still unable to get the idea of inverted masks. IIRC Andei had also not
supported/accepted this masking scheme. But I'll be okay with it if he
supports this masking.

>   d. change the ioctl to be a SCAN with optional WP. Addressing the
> 	original use-case, GetWriteWatch() can be implemented as:
As I've mentioned several times previously (without the name of
ResetWriteWatch()) that we need exclusive WP without GET. This could be
implemented with UFFDIO_WRITEPROTECT. But when we use UFFDIO_WRITEPROTECT,
we hit some special case and performance is very slow. So with UFFD WP
expert, Peter Xu we have decided to put exclusive WP in this IOCTL for
implementation of ResetWriteWatch().

A lot of simplification of the patch is made possible because of not
keeping exclusive WP. (You have also written some quality code, more better.)

> 
> 		memset(&args, 0, sizeof(args));
> 		args.start = lpBaseAddress;
> 		args.end = lpBaseAddress + dwRegionSize;
> 		args.max_pages = *lpdwCount;
> 		*lpdwGranularity = PAGE_SIZE;
> 		args.flags = PM_SCAN_CHECK_WPASYNC;
> 		if (dwFlags & WRITE_WATCH_FLAG_RESET)
> 			args.flags |= PM_SCAN_WP_MATCHED;
> 		args.categories_mask = PAGE_IS_WRITTEN;
> 		args.return_mask = PAGE_IS_WRITTEN;
> 		args.vec = tmp_buffer;
> 		args.vec_len = tmp_buffer_len;
> 		do {
> 			n = ioctl(..., &args);
> 			if (n < 0)
> 				return error(n);
> 			for (page_region *p = ...) {
> 				write page addresses;
> 			}
> 			if (output_full)
> 				return ...;
> 		} while (args.start != args.end);
> 		return ...;
> 
> 	For the CRIU's usecase of live migration this would be similar,
> 	but without using max_pages limit. The other CRIU usecase is to
> 	efficiently dump sparse memory mappings - those could use the
> 	full range of page category filtering as needed in a checkpoint
> 	phase.a
>   e. allow no-op calls
I've no issue with it.

> 
> 2. the implementation:
>   a. gather the page-categorising and write-protecting code in one place;
Agreed.

>   b. optimization: add whole-vma skipping for WP usecase;
I don't know who can benefit from it. Do you have any user in mind? When
the user come of this optimization, this can be added later.

>   c. extracted output limiting code to pagemap_scan_output();
If user passes half THP, current code wouldn't split huge page and WP the
whole THP. We would loose the dirty state of other half huge page. This is
bug. consoliding the output limiting code looks optimal, but we'll need to
same limiting code to detect if full THP hasn't been passed in case of THP
and HugeTLB.

>   d. extracted range coalescing to pagemap_scan_push_range();
My old pagemap_scan_output has become pagemap_scan_push_range().

>   e. extracted THP entry handling to pagemap_scan_thp_entry();
Good. But I didn't found value in seperating it just like other historic
pagemap code.

>   f. added a shortcut for non-WP hugetlb scan; avoids conditional
> 	locking;
Yeah, some if conditions have been removed. But overall did couple of calls
to is_interesting and scan_output functions instead of just one.

>   g. extracted scan buffer handling code out of do_pagemap_scan();
>   h. rework output code to always try to write pending ranges; if EFAULT
> 	is generated it always overwrites the original error code;
> 	(the case of SIGKILL is needlessly trying to write the output
> 	now, but this should be rare case and ignoring it makes the code
> 	not needing a goto)
> 
> PTAL and comment/fix. Compile tested with allyesconfig. For the (unlikely)
> case that the patch is good as is:
> 
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
> Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
> 
> [1] https://lore.kernel.org/lkml/20230713101415.108875-3-usama.anjum@collabora.com/
>   "[PATCH v25 2/5] fs/proc/task_mmu: Implement IOCTL to get and
>    optionally clear info about PTEs"
> 
> Best Regards
> Michał Mirosław
> 
> Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
> ---
>  fs/proc/task_mmu.c      | 649 ++++++++++++++++++++++++++++++++++++++++
>  include/linux/hugetlb.h |   1 +
>  include/uapi/linux/fs.h |  56 ++++
>  mm/hugetlb.c            |   2 +-
>  4 files changed, 707 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index c1e6531cb02a..7d624bafecf9 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -19,6 +19,8 @@
>  #include <linux/shmem_fs.h>
>  #include <linux/uaccess.h>
>  #include <linux/pkeys.h>
> +#include <linux/minmax.h>
> +#include <linux/overflow.h>
>  
>  #include <asm/elf.h>
>  #include <asm/tlb.h>
> @@ -1749,11 +1751,658 @@ static int pagemap_release(struct inode *inode, struct file *file)
>  	return 0;
>  }
>  
> +#define PM_SCAN_CATEGORIES	(PAGE_IS_WPASYNC | PAGE_IS_WRITTEN | \
> +				 PAGE_IS_FILE |	PAGE_IS_PRESENT | \
> +				 PAGE_IS_SWAPPED | PAGE_IS_PFNZERO)
> +#define PM_SCAN_FLAGS		(PM_SCAN_WP_MATCHING | PM_SCAN_CHECK_WPASYNC)
> +
> +struct pagemap_scan_private {
> +	struct pm_scan_arg arg;
> +	unsigned long cur_vma_category;
> +	struct page_region *vec_buf, cur_buf;
> +	unsigned long vec_buf_len, vec_buf_index, found_pages, end_addr;
> +	struct page_region __user* vec_out;
> +};
> +
> +static unsigned long pagemap_page_category(struct vm_area_struct *vma, unsigned long addr, pte_t pte)
> +{
> +	unsigned long categories = 0;
> +
> +	if (pte_present(pte)) {
> +		struct page *page = vm_normal_page(vma, addr, pte);
> +
> +		categories |= PAGE_IS_PRESENT;
> +		if (!pte_uffd_wp(pte))
> +			categories |= PAGE_IS_WRITTEN;
> +		if (page && !PageAnon(page))
> +			categories |= PAGE_IS_FILE;
> +		if (is_zero_pfn(pte_pfn(pte)))
> +			categories |= PAGE_IS_PFNZERO;
> +	} else if (is_swap_pte(pte)) {
> +		swp_entry_t swp = pte_to_swp_entry(pte);
> +
> +		categories |= PAGE_IS_SWAPPED;
> +		if (!pte_swp_uffd_wp_any(pte))
> +			categories |= PAGE_IS_WRITTEN;
> +		if (is_pfn_swap_entry(swp) && !PageAnon(pfn_swap_entry_to_page(swp)))
> +			categories |= PAGE_IS_FILE;
> +	}
> +
> +	return categories;
> +}
> +
> +static 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 unsigned long pagemap_thp_category(pmd_t pmd)
> +{
> +	unsigned long categories = 0;
> +
> +	if (pmd_present(pmd)) {
> +		categories |= PAGE_IS_PRESENT;
> +		if (!pmd_uffd_wp(pmd))
> +			categories |= PAGE_IS_WRITTEN;
> +		if (is_zero_pfn(pmd_pfn(pmd)))
> +			categories |= PAGE_IS_PFNZERO;
> +	} else if (is_swap_pmd(pmd)) {
> +		categories |= PAGE_IS_SWAPPED;
> +		if (!pmd_swp_uffd_wp(pmd))
> +			categories |= PAGE_IS_WRITTEN;
> +	}
> +
> +	return categories;
> +}
> +
> +static 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 /* CONFIG_TRANSPARENT_HUGEPAGE */
> +
> +#ifdef CONFIG_HUGETLB_PAGE
> +static unsigned long pagemap_hugetlb_category(pte_t pte)
> +{
> +	unsigned long categories = 0;
> +
> +	if (pte_present(pte)) {
> +		categories |= PAGE_IS_PRESENT;
> +		if (!huge_pte_uffd_wp(pte))
> +			categories |= PAGE_IS_WRITTEN;
> +		if (!PageAnon(pte_page(pte)))
> +			categories |= PAGE_IS_FILE;
> +		if (is_zero_pfn(pte_pfn(pte)))
> +			categories |= PAGE_IS_PFNZERO;
> +	} else if (is_swap_pte(pte)) {
> +		categories |= PAGE_IS_SWAPPED;
> +		if (!pte_swp_uffd_wp_any(pte))
> +			categories |= PAGE_IS_WRITTEN;
> +	}
> +
> +	return categories;
> +}
> +
> +static 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 /* CONFIG_HUGETLB_PAGE */
> +
> +static bool pagemap_scan_is_interesting_page(unsigned long categories,
> +					     const struct pagemap_scan_private *p)
> +{
> +	categories ^= p->arg.category_inverted;
> +	if ((categories & p->arg.category_mask) != p->arg.category_mask)
> +		return false;
> +	if (p->arg.category_anyof_mask && !(categories & p->arg.category_anyof_mask))
> +		return false;
> +
> +	return true;
> +}
> +
> +static bool pagemap_scan_is_interesting_vma(unsigned long categories,
> +					    const struct pagemap_scan_private *p)
> +{
> +	unsigned long required = p->arg.category_mask & PAGE_IS_WPASYNC;
> +
> +	categories ^= p->arg.category_inverted;
> +	if ((categories & required) != required)
> +		return false;
> +	return true;
> +}
> +
> +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;
> +	unsigned long vma_category = 0;
> +
> +	if (userfaultfd_wp_async(vma) && userfaultfd_wp_use_markers(vma))
> +		vma_category |= PAGE_IS_WPASYNC;
> +	else if (p->arg.flags & PM_SCAN_CHECK_WPASYNC)
> +		return -EPERM;
> +
> +	if (vma->vm_flags & VM_PFNMAP)
> +		return 1;
> +
> +	if (!pagemap_scan_is_interesting_vma(vma_category, p))
> +		return 1;
> +
> +	p->cur_vma_category = vma_category;
> +	return 0;
> +}
> +
> +static bool pagemap_scan_push_range(unsigned long categories,
> +				    struct pagemap_scan_private *p,
> +				    unsigned long addr, unsigned long end)
> +{
> +	struct page_region *cur_buf = &p->cur_buf;
> +
> +	/*
> +	 * When there is no output buffer provided at all, the sentinel values
> +	 * won't match here. There is no other way for `cur_buf->end` to be
> +	 * non-zero other than it being non-empty.
> +	 */
> +	if (addr == cur_buf->end && categories == cur_buf->categories) {
> +		cur_buf->end = end;
> +		return true;
> +	}
> +
> +	if (cur_buf->end) {
> +		if (p->vec_buf_index >= p->vec_buf_len)
> +			return false;
> +
> +		memcpy(&p->vec_buf[p->vec_buf_index], cur_buf,
> +		       sizeof(*p->vec_buf));
> +		++p->vec_buf_index;
> +	}
> +
> +	cur_buf->start = addr;
> +	cur_buf->end = end;
> +	cur_buf->categories = categories;
> +	return true;
> +}
> +
> +static void pagemap_scan_backout_range(struct pagemap_scan_private *p,
> +				       unsigned long addr, unsigned long end)
> +{
> +	struct page_region *cur_buf = &p->cur_buf;
> +
> +	if (cur_buf->start != addr) {
> +		cur_buf->end = addr;
> +	} else {
> +		cur_buf->start = cur_buf->end = 0;
> +	}
> +
> +	p->end_addr = 0;
> +}
> +
> +static int pagemap_scan_output(unsigned long categories,
> +			       struct pagemap_scan_private *p,
> +			       unsigned long addr, unsigned long *end)
> +{
> +	unsigned long n_pages, total_pages;
> +	int ret = 0;
> +
> +	if (!p->arg.return_mask)
> +		return 0;
> +
> +	categories &= p->arg.return_mask;
> +
> +	n_pages = (*end - addr) / PAGE_SIZE;
> +	if (check_add_overflow(p->found_pages, n_pages, &total_pages) || total_pages > p->arg.max_pages) {
> +		size_t n_too_much = total_pages - p->arg.max_pages;
> +		*end -= n_too_much * PAGE_SIZE;
> +		n_pages -= n_too_much;
> +		ret = -ENOSPC;
> +	}
> +
> +	if (!pagemap_scan_push_range(categories, p, addr, *end)) {
> +		*end = addr;
> +		n_pages = 0;
> +		ret = -ENOSPC;
> +	}
> +
> +	p->found_pages += n_pages;
> +	if (ret)
> +		p->end_addr = *end;
> +	return ret;
> +}
> +
> +static int pagemap_scan_thp_entry(pmd_t *pmd, unsigned long start,
> +				  unsigned long end, struct mm_walk *walk)
> +{
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	struct pagemap_scan_private *p = walk->private;
> +	struct vm_area_struct *vma = walk->vma;
> +	unsigned long categories;
> +	spinlock_t *ptl;
> +	int ret = 0;
> +
> +	ptl = pmd_trans_huge_lock(pmd, vma);
> +	if (!ptl)
> +		return -ENOENT;
> +
> +	categories = p->cur_vma_category | pagemap_thp_category(*pmd);
> +
> +	if (!pagemap_scan_is_interesting_page(categories, p))
> +		goto out_unlock;
> +
> +	ret = pagemap_scan_output(categories, p, start, &end);
> +	if (start == end)
> +		goto out_unlock;
> +
> +	if (~p->arg.flags & PM_SCAN_WP_MATCHING)
> +		goto out_unlock;
> +	if (~categories & PAGE_IS_WRITTEN)
> +		goto out_unlock;
> +
> +	/*
> +	 * Break huge page into small pages if the WP operation
> +	 * need to be performed is on a portion of the huge page.
> +	 */
> +	if (ret == -ENOSPC) {
> +		spin_unlock(ptl);
> +		split_huge_pmd(vma, pmd, start);
> +		pagemap_scan_backout_range(p, start, end);
> +		return -ENOENT;
> +	}
> +
> +	make_uffd_wp_pmd(vma, start, pmd);
> +	flush_tlb_range(vma, start, end);
> +out_unlock:
> +	spin_unlock(ptl);
> +	return ret;
> +#else /* !CONFIG_TRANSPARENT_HUGEPAGE */
> +	return -ENOENT;
> +#endif
> +}
> +
> +static 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;
> +	pte_t *pte, *start_pte;
> +	unsigned long addr;
> +	bool flush = false;
> +	spinlock_t *ptl;
> +	int ret;
> +
> +	arch_enter_lazy_mmu_mode();
> +
> +	ret = pagemap_scan_thp_entry(pmd, start, end, walk);
> +	if (ret != -ENOENT) {
> +		arch_leave_lazy_mmu_mode();
> +		return ret;
> +	}
> +
> +	start_pte = pte = pte_offset_map_lock(vma->vm_mm, pmd, start, &ptl);
> +	if (!pte) {
> +		arch_leave_lazy_mmu_mode();
> +		walk->action = ACTION_AGAIN;
> +		return 0;
> +	}
> +
> +	for (addr = start; addr != end; pte++, addr += PAGE_SIZE) {
> +		unsigned long categories = p->cur_vma_category |
> +			pagemap_page_category(vma, addr, ptep_get(pte));
> +		unsigned long next = addr + PAGE_SIZE;
> +
> +		if (!pagemap_scan_is_interesting_page(categories, p))
> +			continue;
> +
> +		ret = pagemap_scan_output(categories, p, addr, &next);
> +		if (next == addr)
> +			break;
> +
> +		if (~p->arg.flags & PM_SCAN_WP_MATCHING)
> +			continue;
> +		if (~categories & PAGE_IS_WRITTEN)
> +			continue;
> +
> +		make_uffd_wp_pte(vma, addr, pte);
> +		if (!flush) {
> +			start = addr;
> +			flush = true;
> +		}
> +	}
> +
> +	if (flush)
> +		flush_tlb_range(vma, start, addr);
> +
> +	pte_unmap_unlock(start_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;
> +	unsigned long categories;
> +	spinlock_t *ptl;
> +	int ret = 0;
> +	pte_t pte;
> +
> +	if (~p->arg.flags & PM_SCAN_WP_MATCHING) {
> +		/* Go the short route when not write-protecting pages. */
> +
> +		pte = huge_ptep_get(ptep);
> +		categories = p->cur_vma_category | pagemap_hugetlb_category(pte);
> +
> +		if (!pagemap_scan_is_interesting_page(categories, p))
> +			return 0;
> +
> +		return pagemap_scan_output(categories, p, start, &end);
> +	}
> +
> +	if (n_pages < HPAGE_SIZE/PAGE_SIZE) {
> +		/*
> +		 * Partial hugetlb page clear isn't supported
> +		 */
> +		p->end_addr = start;
> +		return -EINVAL;
> +	}
> +
> +	i_mmap_lock_write(vma->vm_file->f_mapping);
> +	ptl = huge_pte_lock(hstate_vma(vma), vma->vm_mm, ptep);
> +
> +	pte = huge_ptep_get(ptep);
> +	categories = p->cur_vma_category | pagemap_hugetlb_category(pte);
> +
> +	if (!pagemap_scan_is_interesting_page(categories, p))
> +		goto out_unlock;
> +
> +	ret = pagemap_scan_output(categories, p, start, &end);
> +	if (start == end)
> +		goto out_unlock;
> +
> +	if (categories & PAGE_IS_WRITTEN) {
> +		make_uffd_wp_huge_pte(vma, start, ptep, pte);
> +		flush_hugetlb_tlb_range(vma, start, end);
> +	}
> +
> +out_unlock:
> +	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)
> +{
> +	struct pagemap_scan_private *p = walk->private;
> +	struct vm_area_struct *vma = walk->vma;
> +	int ret;
> +
> +	if (!vma || !pagemap_scan_is_interesting_page(p->cur_vma_category, p))
> +		return 0;
> +
> +	ret = pagemap_scan_output(p->cur_vma_category, p, addr, &end);
> +	if (addr == end)
> +		return ret;
> +
> +	if (~p->arg.flags & PM_SCAN_WP_MATCHING)
> +		return ret;
> +
> +	int err = uffd_wp_range(vma, addr, end - addr, true);
> +	if (err < 0)
> +		ret = err;
> +
> +	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_get_args(struct pm_scan_arg *arg,
> +				 unsigned long uarg)
> +{
> +	unsigned long start, end, vec;
> +
> +	if (copy_from_user(arg, (void __user *)uarg, sizeof(*arg)))
> +		return -EFAULT;
> +
> +	if (arg->size != sizeof(struct pm_scan_arg))
> +		return -EINVAL;
> +
> +	/* Validate requested features */
> +	if (arg->flags & ~PM_SCAN_FLAGS)
> +		return -EINVAL;
> +	if ((arg->category_inverted | arg->category_mask |
> +	     arg->category_anyof_mask | arg->return_mask) & ~PM_SCAN_CATEGORIES)
> +		return -EINVAL;
> +
> +	start = untagged_addr((unsigned long)arg->start);
> +	end = untagged_addr((unsigned long)arg->end);
> +	vec = untagged_addr((unsigned long)arg->vec);
> +
> +	/* Validate memory pointers */
> +	if (!IS_ALIGNED(start, PAGE_SIZE))
> +		return -EINVAL;
> +	if (!access_ok((void __user *)start, end - start))
> +		return -EFAULT;
> +	if (arg->vec_len && !vec)
> +		return -EFAULT;
> +	if (!access_ok((void __user *)vec,
> +		       arg->vec_len * sizeof(struct page_region)))
> +		return -EFAULT;
> +
> +	/* Fixup default values */
> +	if (!arg->max_pages)
> +		arg->max_pages = ULONG_MAX;
> +
> +	return 0;
> +}
> +
> +static int pagemap_scan_writeback_args(struct pm_scan_arg *arg,
> +				       unsigned long uargl)
> +{
> +	struct pm_scan_arg __user *uarg	= (void __user *)uargl;
> +
> +	if (copy_to_user(&uarg->start, &arg->start, sizeof(arg->start)))
> +		return -EFAULT;
> +
> +	return 0;
> +}
> +
> +static int pagemap_scan_init_bounce_buffer(struct pagemap_scan_private *p)
> +{
> +	if (!p->arg.vec_len) {
> +		/*
> +		 * An arbitrary non-page-aligned sentinel value for
> +		 * pagemap_scan_push_range().
> +		 */
> +		p->cur_buf.start = p->cur_buf.end = ULLONG_MAX;
> +		return 0;
> +	}
> +
> +	/*
> +	 * Allocate a smaller buffer to get output from inside the page
> +	 * walk functions and walk the range in PAGEMAP_WALK_SIZE chunks.
> +	 * The last range is always stored in p.cur_buf to allow coalescing
> +	 * consecutive ranges that have the same categories returned across
> +	 * walk_page_range() calls.
> +	 */
> +	p->vec_buf_len = min_t(size_t, PAGEMAP_WALK_SIZE >> PAGE_SHIFT,
> +			       p->arg.vec_len - 1);
> +	p->vec_buf = kmalloc_array(p->vec_buf_len, sizeof(*p->vec_buf),
> +				   GFP_KERNEL);
> +	if (!p->vec_buf)
> +		return -ENOMEM;
> +
> +	p->vec_out = (void __user *)p->arg.vec;
> +
> +	return 0;
> +}
> +
> +static int pagemap_scan_flush_buffer(struct pagemap_scan_private *p)
> +{
> +	const struct page_region *buf = p->vec_buf;
> +	int n = (int)p->vec_buf_index;
> +
> +	if (!n)
> +		return 0;
> +
> +	if (copy_to_user(p->vec_out, buf, n * sizeof(*buf)))
> +		return -EFAULT;
> +
> +	p->arg.vec_len -= n;
> +	p->vec_out += n;
> +
> +	p->vec_buf_index = 0;
> +	p->vec_buf_len = min_t(size_t, p->vec_buf_len, p->arg.vec_len - 1);
> +
> +	return n;
> +}
> +
> +static long do_pagemap_scan(struct mm_struct *mm, unsigned long uarg)
> +{
> +	unsigned long walk_start, walk_end;
> +	struct mmu_notifier_range range;
> +	struct pagemap_scan_private p;
> +	size_t n_ranges_out = 0;
> +	int ret;
> +
> +	memset(&p, 0, sizeof(p));
> +	ret = pagemap_scan_get_args(&p.arg, uarg);
> +	if (ret)
> +		return ret;
> +
> +	ret = pagemap_scan_init_bounce_buffer(&p);
> +	if (ret)
> +		return ret;
> +
> +	/* Protection change for the range is going to happen. */
> +	if (p.arg.flags & PM_SCAN_WP_MATCHING) {
> +		mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_VMA, 0,
> +					mm, p.arg.start, p.arg.end);
> +		mmu_notifier_invalidate_range_start(&range);
> +	}
> +
> +	walk_start = walk_end = p.arg.start;
> +	for (; walk_end != p.arg.end; walk_start = walk_end) {
> +		int n_out;
> +		walk_end = min_t(unsigned long,
> +				 (walk_start + PAGEMAP_WALK_SIZE) & PAGEMAP_WALK_MASK,
> +				 p.arg.end);
> +
> +		ret = mmap_read_lock_killable(mm);
> +		if (ret)
> +			break;
> +		ret = walk_page_range(mm, walk_start, walk_end,
> +				      &pagemap_scan_ops, &p);
> +		mmap_read_unlock(mm);
> +
> +		n_out = pagemap_scan_flush_buffer(&p);
> +		if (n_out < 0)
> +			ret = n_out;
> +		else
> +			n_ranges_out += n_out;
> +
> +		if (ret)
> +			break;
> +	}
> +
> +	if (p.cur_buf.start != p.cur_buf.end) {
> +		if (copy_to_user(p.vec_out, &p.cur_buf, sizeof(p.cur_buf)))
> +			ret = -EFAULT;
> +		else
> +			++n_ranges_out;
> +	}
> +
> +	/* ENOSPC signifies early stop (buffer full) from the walk. */
> +	if (!ret || ret == -ENOSPC)
> +		ret = n_ranges_out;
> +
> +	p.arg.start = p.end_addr ? p.end_addr : walk_start;
> +	if (pagemap_scan_writeback_args(&p.arg, uarg))
> +		ret = -EFAULT;
> +
> +	if (p.arg.flags & PM_SCAN_WP_MATCHING)
> +		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 9f4bac3df59e..4f5fed605538 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -259,6 +259,7 @@ 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..db39442befcb 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -305,4 +305,60 @@ 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_WPASYNC		(1 << 0)
> +#define PAGE_IS_WRITTEN		(1 << 1)
> +#define PAGE_IS_FILE		(1 << 2)
> +#define PAGE_IS_PRESENT		(1 << 3)
> +#define PAGE_IS_SWAPPED		(1 << 4)
> +#define PAGE_IS_PFNZERO		(1 << 5)
> +
> +/*
> + * struct page_region - Page region with flags
> + * @start:	Start of the region
> + * @end:	End of the region (exclusive)
> + * @categories:	PAGE_IS_* category bitmask for the region
> + */
> +struct page_region {
> +	__u64 start;
> +	__u64 end;
> +	__u64 categories;
> +};
> +
> +/* Flags for PAGEMAP_SCAN ioctl */
> +#define PM_SCAN_WP_MATCHING	(1 << 0)	/* Write protect the pages matched. */
> +#define PM_SCAN_CHECK_WPASYNC	(1 << 1)	/* Abort the scan when a non-WP-enabled page is found. */
> +
> +/*
> + * struct pm_scan_arg - Pagemap ioctl argument
> + * @size:		Size of the structure
> + * @flags:		Flags for the IOCTL
> + * @start:		Starting address of the region
> + *			(Ending address of the walk is also returned in it)
> + * @end:		Ending address of the region
> + * @vec:		Address of page_region struct array for output
> + * @vec_len:		Length of the page_region struct array
> + * @max_pages:		Optional limit for number of returned pages (0 = disabled)
> + * @category_inverted:	PAGE_IS_* categories which values match if 0 instead of 1
> + * @category_mask:	Skip pages for which any category doesn't match
> + * @category_anyof_mask: Skip pages for which no category matches
> + * @return_mask:	PAGE_IS_* categories that are to be reported in `page_region`s returned
> + */
> +struct pm_scan_arg {
> +	__u64 size;
> +	__u64 flags;
> +	__u64 start;
> +	__u64 end;
> +	__u64 vec;
> +	__u64 vec_len;
> +	__u64 max_pages;
> +	__u64 category_inverted;
> +	__u64 category_mask;
> +	__u64 category_anyof_mask;
> +	__u64 return_mask;
> +};
> +
>  #endif /* _UAPI_LINUX_FS_H */
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 751968eff92d..4bf8cc4aba2e 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5008,7 +5008,7 @@ 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;
>  

-- 
BR,
Muhammad Usama Anjum