by Muhammad Usama Anjum

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

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

2023-06-23 09:52:23

by Michał Mirosław

[permalink] [raw]

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

On Thu, 22 Jun 2023 at 12:21, Muhammad Usama Anjum
<[email protected]> wrote:
> On 6/22/23 12:45 AM, Andrei Vagin wrote:
> > On Wed, Jun 21, 2023 at 11:34:54AM +0500, Muhammad Usama Anjum wrote:
> >> On 6/20/23 11:03 PM, Andrei Vagin wrote:
[...]
> >>> 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.

It would be great if the ioctl returned the address the walk finished
at. This would remove the special case for "buffer full after full
page was added" and if it happens that despite `max_pages` limit was
reached but no more pages would need to be added the caller would not
have to call the ioctl again on the remaining range.

[...]
> >>> 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.

I don't think we should treat mincore() as a good API example. Its
interface has similar performance problems to what select() or poll()
does. In this ioctl's case, we can limit the output at this end (large
anyway) as it won't affect the performance if for x TiB of memory the
call is made twice instead of once. (Returning the end of the walk
reached would be much help here.)

Best Regards
Michał Mirosław