Re: [v2] fs/proc/task_mmu: Implement IOCTL for efficient page table scanning

["Michał Mirosław" <emmir@google.com>]

On Mon, 24 Jul 2023 at 16:04, Muhammad Usama Anjum
<usama.anjum@collabora.com> wrote:
>
> Fixed found bugs. Testing it further.
>
> - Split and backoff in case buffer full case as well
> - Fix the wrong breaking of loop if page isn't interesting, skip intead
> - Untag the address and save them into struct
> - Round off the end address to next page
>
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
> ---
>  fs/proc/task_mmu.c | 54 ++++++++++++++++++++++++++--------------------
>  1 file changed, 31 insertions(+), 23 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index add21fdf3c9a..64b326d0ec6d 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1985,18 +1989,19 @@ static int pagemap_scan_output(unsigned long
> categories,
>         unsigned long n_pages, total_pages;
>         int ret = 0;
>
> +       if (!p->vec_buf)
> +               return 0;
> +
>         if (!pagemap_scan_is_interesting_page(categories, p)) {
>                 *end = addr;
>                 return 0;
>         }
>
> -       if (!p->vec_buf)
> -               return 0;
> -
>         categories &= p->arg.return_mask;

This is wrong - is_interesting() check must happen before output as
the `*end = addr` means the range should be skipped, but return 0
requests continuing of the walk.

> @@ -2044,7 +2050,7 @@ static int pagemap_scan_thp_entry(pmd_t *pmd,
> unsigned long start,
>          * Break huge page into small pages if the WP operation
>          * need to be performed is on a portion of the huge page.
>          */
> -       if (end != start + HPAGE_SIZE) {
> +       if (end != start + HPAGE_SIZE || ret == -ENOSPC) {

Why is it needed? If `end == start + HPAGE_SIZE` then we're handling a
full hugepage anyway.

> @@ -2066,8 +2072,8 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd,
> unsigned long start,
>  {
>         struct pagemap_scan_private *p = walk->private;
>         struct vm_area_struct *vma = walk->vma;
> +       unsigned long addr, categories, next;
>         pte_t *pte, *start_pte;
> -       unsigned long addr;
>         bool flush = false;
>         spinlock_t *ptl;
>         int ret;
> @@ -2088,12 +2094,14 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd,
> unsigned long start,
>         }
>
>         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;
> +               categories = p->cur_vma_category |
> +                            pagemap_page_category(vma, addr, ptep_get(pte));
> +               next = addr + PAGE_SIZE;

Why moving the variable declarations out of the loop?

>
>                 ret = pagemap_scan_output(categories, p, addr, &next);
> -               if (next == addr)
> +               if (ret == 0 && next == addr)
> +                       continue;
> +               else if (next == addr)
>                         break;

Ah, this indeed was a bug. Nit:

if (next == addr) { if (!ret) continue; break; }

> @@ -2204,8 +2212,6 @@ static const struct mm_walk_ops pagemap_scan_ops = {
>  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;
>
> @@ -2219,22 +2225,24 @@ static int pagemap_scan_get_args(struct pm_scan_arg
> *arg,
>              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);
> +       arg->start = untagged_addr((unsigned long)arg->start);
> +       arg->end = untagged_addr((unsigned long)arg->end);
> +       arg->vec = untagged_addr((unsigned long)arg->vec);

BTW, We should we keep the tag in args writeback().

>         /* Validate memory pointers */
> -       if (!IS_ALIGNED(start, PAGE_SIZE))
> +       if (!IS_ALIGNED(arg->start, PAGE_SIZE))
>                 return -EINVAL;
> -       if (!access_ok((void __user *)start, end - start))
> +       if (!access_ok((void __user *)arg->start, arg->end - arg->start))
>                 return -EFAULT;
> -       if (!vec && arg->vec_len)
> +       if (!arg->vec && arg->vec_len)
>                 return -EFAULT;
> -       if (vec && !access_ok((void __user *)vec,
> +       if (arg->vec && !access_ok((void __user *)arg->vec,
>                               arg->vec_len * sizeof(struct page_region)))
>                 return -EFAULT;
>
>         /* Fixup default values */
> +       arg->end = (arg->end & ~PAGE_MASK) ?
> +                  ((arg->end & PAGE_MASK) + PAGE_SIZE) : (arg->end);

arg->end = ALIGN(arg->end, PAGE_SIZE);

>         if (!arg->max_pages)
>                 arg->max_pages = ULONG_MAX;
>

Best Regards
Michał Mirosław