On 6/27/23 7:36 PM, Andrei Vagin wrote:
> On Tue, Jun 27, 2023 at 02:00:31PM +0500, Muhammad Usama Anjum wrote:
>> Hi Andrei and Michal,
>>
>> Lets resolve last two points. Please reply below.
>>
>> On 6/27/23 6:46 AM, Andrei Vagin wrote:
>> ...
>>>> +#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 (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) {
>>>
>>> should it be done only if is_interesting is set?
>> This can be good optimization. We shouldn't return error before finding if
>> page is interesting. I'll update.
>>
>>>
>>>> + ret = PM_SCAN_END_WALK;
>>>> + goto unlock_and_return;
>>>> + }
>>>> +
>>>> + bitmap = PM_SCAN_FLAGS(is_written, pagemap_scan_is_huge_file(ptent),
>>>> + pte_present(ptent), is_swap_pte(ptent),
>>>> + pte_present(ptent) && is_zero_pfn(pte_pfn(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;
>>>> +}
>> ...
>>>> +
>>>> +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);
>>>> + 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) ? 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);
>>>> + }
>>>> +
>>>> + walk_end = (walk_start + PAGEMAP_WALK_SIZE) & PAGEMAP_WALK_MASK;
>>>> + if (walk_end > end)
>>>> + walk_end = end;
>>>> +
>>>
>>> If this loop can run for a long time, we need to interrupt it in case of
>>> pending signals.
>>>
>>> If you think we don't need to do that, pls explain in the commit
>>> message, so that maintainers don't miss this part and double check that
>>> everything is alright here.
>> This can be done. I'll add to the commit message that we are walking over
>> entire range passed.
>>
>>>
>>>> + ret = mmap_read_lock_killable(mm);
>>>> + if (ret)
>>>
>>> If any pages have been handled, we need to report them to user-space. It
>>> isn't acceptable to return a error in such cases.
>> This will return error only when task has gotten some serios signal and it
>> is giong to be killed. In this scenerio, we shouldn't care about returning
>> gracefully. Why do you think we should return gracefully in this case?
>
> You are right, it can be interrupted only by a fatal signal. You can
> ignore this comment.
>
>>
>>>
>>> And we need to report an address where it stopped scanning.
>>> We can do that by adding zero length vector.
>> I don't want to do multiplexing the ending address in vec. Can we add
>> end_addr variable in struct pm_scan_arg to always return the ending address?
>>
>> struct pm_scan_arg {
>> ...
>> _u64 end_addr;
>> };
>>
>>
>>>
>>>
>>>> + goto free_data;
>>>> + 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 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;
>>>
>>> Should we set ret to zero here if it is equal PM_SCAN_END_WALK.
>> No, PM_SCAN_END_WALK is just internal code to stop the page walk and return
>> immedtitely. When we get this return value, we stop this loop and return to
>> user with whatever data we have in user buffer.
>
> but PM_SCAN_END_WALK is returned when p.vec_buf is full, so we can
> restart the loop after coping vec_buf to the user buffer, can't we?
No, we set the capacity of p.vec_buf based on how many empty slots are
remaining in user buffer. So when p.vec_buf is marked as full, it means
user buffer is full. S
>
>>
>>>
>>>> + }
>>>> + }
>>>> +
>>>> + 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;
>>>> +}
>>>> +
>> ...
>>
>> --
>> BR,
>> Muhammad Usama Anjum
--
BR,
Muhammad Usama Anjum
