linux-kernel - Re: [PATCH v22 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

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Date:   Mon, 3 Jul 2023 11:47:37 +0500
From:   Muhammad Usama Anjum <usama.anjum@...labora.com>
To:     Andrei Vagin <avagin@...il.com>
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>, Danylo Mocherniuk <mdanylo@...gle.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.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>,
        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 v22 2/5] fs/proc/task_mmu: Implement IOCTL to get and
 optionally clear info about PTEs

On 6/30/23 8:01 PM, Andrei Vagin wrote:
> On Wed, Jun 28, 2023 at 02:54:23PM +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), swapped
>>   (PAGE_IS_SWAPPED) or page has pfn zero (PAGE_IS_PFNZERO).
>> - 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. The
>> entire address range passed by user [start, end) is scanned until either
>> the user provided buffer is full or max_pages have been found.
>>
>> Signed-off-by: Muhammad Usama Anjum <usama.anjum@...labora.com>
> 
> <snip>
> 
>> +
>> +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 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);
>> +	end = untagged_addr((unsigned long)arg.end);
>> +	vec = (struct page_region __user *)untagged_addr((unsigned long)arg.vec);
>> +
>> +	ret = pagemap_scan_args_valid(&arg, start, end, vec);
>> +	if (ret)
>> +		return ret;
>> +
>> +	p.max_pages = (arg.max_pages) ? arg.max_pages : ULONG_MAX;
>> +	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) {
>> +		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);
>> +		}
>> +
>> +		ret = mmap_read_lock_killable(mm);
>> +		if (ret)
>> +			goto return_status;
>> +
>> +		walk_end = min((walk_start + PAGEMAP_WALK_SIZE) & PAGEMAP_WALK_MASK, end);
>> +
>> +		ret = walk_page_range(mm, walk_start, walk_end,
>> +				      &pagemap_scan_ops, &p);
>> +		mmap_read_unlock(mm);
>> +
>> +		if (ret && ret != PM_SCAN_FOUND_MAX_PAGES &&
>> +		    ret != PM_SCAN_END_WALK)
>> +			goto return_status;
>> +
>> +		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 return_status;
>> +			}
>> +			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 return_status;
>> +		}
>> +		vec_index++;
>> +	}
>> +
>> +	ret = vec_index;
>> +
>> +return_status:
>> +	arg.start = (unsigned long)walk_end;
> 
> This doesn't look right. pagemap_scan_pmd_entry can stop early. For
> example, it can happen when it hits the max_pages limit. Do I miss
> something?
The walk_page_range() calls pagemap_scan_pmd_entry(). So whatever status is
returned from pagemap_scan_pmd_entry(), walk_page_range() returns to this
function where we are handling the status code. After while loop starts,
there is only 1 return path. Hence there isn't any path missing where we'll
miss setting arg.start.

> 
>> +	if (copy_to_user(&uarg->start, &arg.start, sizeof(arg.start)))
>> +		ret = -EFAULT;
>> +
>> +	if (IS_PM_SCAN_WP(p.flags))
>> +		mmu_notifier_invalidate_range_end(&range);
>> +
>> +	kfree(p.vec_buf);
>> +	return ret;
>> +}
>> +

-- 
BR,
Muhammad Usama Anjum