On Thu, 6 Apr 2023 at 09:40, Muhammad Usama Anjum
<usama.anjum@xxxxxxxxxxxxx> 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 write protect the pages
> (atomic PAGE_IS_WRITTEN + PAGEMAP_WP_ENGAGE)
>
> This IOCTL can be extended to get information about more PTE bits.
[...]
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
[...]
> +static inline bool is_pte_uffd_wp(pte_t pte)
> +{
> + return ((pte_present(pte) && pte_uffd_wp(pte)) ||
> + pte_swp_uffd_wp_any(pte));
Nit: outer parentheses are not needed for `return`ed value -- please
remove. (Same in other places.)
> @@ -1768,11 +1789,416 @@ static int pagemap_release(struct inode *inode, struct file *file)
> return 0;
> }
>
> +#define PM_SCAN_FOUND_MAX_PAGES (1)
> +#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 PM_SCAN_OP_IS_WP(a) (a->flags & PM_SCAN_OP_WP)
Nit: PM_SCAN_DO_UFFD_WP()? It would shift the hint in the name from
what op is executed to what behaviour is requested.
> +#define PM_SCAN_BITMAP(wt, file, present, swap) \
> + (wt | file << 1 | present << 2 | swap << 3)
Please parenthesize macro arguments ("(wt)", "(file)", etc.) to not
have to worry about operator precedence when passed a complex
expression.
I
[...]
> +static inline bool pagemap_scan_is_written_set(struct pagemap_scan_private *p)
pagemap_scan_checks_page_written? or similar 'scan is written' doesn't
seem to convey the expected intention.
The function is used only once in ..._test_walk(), so maybe just
inline, possibly using a temporary `bool` to make the condition easier
to read?
[...]
> +static int pagemap_scan_output(bool wt, bool file, bool pres, bool swap,
Could you try out how the code would look when passing the bitmap
instead of separate booleans?
> + struct pagemap_scan_private *p,
> + unsigned long addr, unsigned int n_pages)
> +{
[...]
> + if ((cur->start + cur->len * PAGE_SIZE == addr) &&
> + (cur->bitmap == bitmap)) {
Nit: bitmap check is cheaper, so I'd put it first. BTW, inner
parentheses are not needed here.
> + cur->len += n_pages;
> + p->found_pages += n_pages;
> +
> + if (p->max_pages && (p->found_pages == p->max_pages))
> + return PM_SCAN_FOUND_MAX_PAGES;
> +
> + return 0;
> + }
> +
> + if (!p->vec_index || ((p->vec_index + 1) < p->vec_len)) {
It looks that `if (p->vec_index < p->vec_len)` is enough here - if we
have vec_len == 0 here, then we'd not fit the entry in the userspace
buffer anyway. Am I missing something?
> +
> + if (cur->len) {
> + memcpy(&p->vec[p->vec_index], cur,
> + sizeof(struct page_region));
> + p->vec_index++;
> + }
> + cur->start = addr;
> + cur->len = n_pages;
> + cur->bitmap = bitmap;
> + p->found_pages += n_pages;
> +
> + if (p->max_pages && (p->found_pages == p->max_pages))
> + return PM_SCAN_FOUND_MAX_PAGES;
> +
> + return 0;
> + }
> +
> + return -ENOSPC;
> +}
> +
> +static inline int pagemap_scan_deposit(struct pagemap_scan_private *p,
> + struct page_region __user *vec,
> + unsigned long *vec_index)
..._deposit() is used only in single place - please inline.
[...]
> +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;
> + bool is_written, is_file, is_present, is_swap;
> + struct vm_area_struct *vma = walk->vma;
> + unsigned long addr = end;
> + spinlock_t *ptl;
> + int ret = 0;
> + pte_t *pte;
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> + ptl = pmd_trans_huge_lock(pmd, vma);
> + if (ptl) {
Nit: `page_lock` or `pt_lock` to make it easier to guess the purpose?
> + unsigned long n_pages = (end - start)/PAGE_SIZE;
> +
> + is_written = !is_pmd_uffd_wp(*pmd);
> + is_file = vma->vm_file;
> + is_present = pmd_present(*pmd);
> + is_swap = is_swap_pmd(*pmd);
> +
> + spin_unlock(ptl);
> +
> + /*
> + * Break huge page into small pages if the WP operation need to
> + * be performed is on a portion of the huge page or if max_pages
> + * pages limit would exceed.
BTW, could the `max_pages` limit be relaxed a bit (in that it would be
possible to return more pages if they all merge into the last vector
entry) so that it would not need to split otherwise-matching huge
page? It would remove the need for this special handling in the kernel
and splitting the page by this read-only-appearing ioctl?
> + */
> + if (is_written && PM_SCAN_OP_IS_WP(p) &&
> + ((end - start < HPAGE_SIZE) ||
> + (p->max_pages &&
> + (p->max_pages - p->found_pages) < n_pages))) {
> +
> + split_huge_pmd(vma, pmd, start);
> + goto process_smaller_pages;
> + }
> +
> + if (p->max_pages &&
> + p->found_pages + n_pages > p->max_pages)
> + n_pages = p->max_pages - p->found_pages;
> +
> + ret = pagemap_scan_output(is_written, is_file, is_present,
> + is_swap, p, start, n_pages);
> + if (ret < 0)
> + return ret;
> +
> + if (is_written && PM_SCAN_OP_IS_WP(p) &&
> + uffd_wp_range(vma, start, HPAGE_SIZE, true) < 0)
> + ret = -EINVAL;
Why not propagate the error from uffd_wp_range()?
[...]
> +static long do_pagemap_cmd(struct mm_struct *mm,
> + struct pm_scan_arg __user *uarg)
Please rename the function to `do_pagemap_scan` as it implements just
this single ioctl now.
> +{
[...]
> + start = (unsigned long)untagged_addr(arg.start);
> + vec = (struct page_region *)(unsigned long)untagged_addr(arg.vec);
Is the inner cast needed?
> + 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.flags = arg.flags;
> + 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.cur.len = 0;
> + p.cur.start = 0;
> + p.vec = NULL;
> + p.vec_len = (PAGEMAP_WALK_SIZE >> PAGE_SHIFT);
Nit: parentheses are not needed here, please remove.
> +
> + /*
> + * 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 between different walks and
> + * store the p.cur at the end of the walk to the user buffer.
> + */
> + p.vec = kmalloc_array(p.vec_len, sizeof(struct page_region),
> + GFP_KERNEL);
> + if (!p.vec)
> + return -ENOMEM;
> +
> + walk_start = walk_end = start;
> + while (walk_end < end && !ret) {
The loop will stop if a previous iteration returned ENOSPC (and the
error will be lost) - is it intended?
> + p.vec_index = 0;
> +
> + empty_slots = arg.vec_len - vec_index;
> + p.vec_len = min(p.vec_len, empty_slots);
> +
> + walk_end = (walk_start + PAGEMAP_WALK_SIZE) & PAGEMAP_WALK_MASK;
> + if (walk_end > end)
> + walk_end = end;
> +
> + mmap_read_lock(mm);
> + ret = walk_page_range(mm, walk_start, walk_end,
> + &pagemap_scan_ops, &p);
> + mmap_read_unlock(mm);
> +
> + if (ret && ret != -ENOSPC && ret != PM_SCAN_FOUND_MAX_PAGES)
> + goto free_data;
> +
> + walk_start = walk_end;
> + if (p.vec_index) {
> + if (copy_to_user(&vec[vec_index], p.vec,
> + p.vec_index *
> + sizeof(struct page_region))) {
sizeof(*p.vec) ?
> + ret = -EFAULT;
> + goto free_data;
> + }
> + vec_index += p.vec_index;
> + }
> + }
> + ret = pagemap_scan_deposit(&p, vec, &vec_index);
> + if (!ret)
> + ret = vec_index;
> +free_data:
> + kfree(p.vec);
> +
> + return ret;
> +}
> +
> +static long pagemap_scan_ioctl(struct file *file, unsigned int cmd,
> + unsigned long arg)
> +{
> + struct pm_scan_arg __user *uarg = (struct pm_scan_arg __user *)arg;
This is specific to PAGEMAP_SCAN ioctl, so should go into do_pagemap_cmd().
> + struct mm_struct *mm = file->private_data;
> +
> + switch (cmd) {
> + case PAGEMAP_SCAN:
> + return do_pagemap_cmd(mm, uarg);
[...]
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -210,6 +210,14 @@ extern bool userfaultfd_wp_async(struct vm_area_struct *vma);
>
> #else /* CONFIG_USERFAULTFD */
>
> +static inline long uffd_wp_range(struct mm_struct *dst_mm,
> + struct vm_area_struct *vma,
> + unsigned long start, unsigned long len,
> + bool enable_wp)
> +{
> + return 0;
> +}
> +
> /* mm helpers */
> static inline vm_fault_t handle_userfault(struct vm_fault *vmf,
> unsigned long reason)
Shouldn't this part be in the patch introducing uffd_wp_range()?
Best Regards
Michał Mirosław
