Re: [PATCH v27 2/6] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

[Andrei Vagin]

On Tue, Aug 8, 2023 at 12:35 PM Muhammad Usama Anjum
<usama.anjum@xxxxxxxxxxxxx> wrote:
>
> On 8/9/23 12:21 AM, Andrei Vagin wrote:
> > On Tue, Aug 8, 2023 at 3:43 AM Muhammad Usama Anjum
> > <usama.anjum@xxxxxxxxxxxxx> wrote:
> >
> > ....
> >
> >> +static int pagemap_scan_output(unsigned long categories,
> >> +                              struct pagemap_scan_private *p,
> >> +                              unsigned long addr, unsigned long *end)
> >> +{
> >> +       unsigned long n_pages, total_pages;
> >> +       int ret = 0;
> >> +
> >> +       if (!p->vec_buf)
> >> +               return 0;
> >> +
> >> +       categories &= p->arg.return_mask;
> >> +
> >> +       n_pages = (*end - addr) / PAGE_SIZE;
> >> +       if (check_add_overflow(p->found_pages, n_pages, &total_pages) || //TODO
> >
> > Need to fix this TODO.
> Sorry, I forgot to remove the "//TODO". As far as I've understood, the last
> discussion ended in keeping the check_add_overflow(). [1] I'll just remove
> the TODO.
>
> https://lore.kernel.org/all/CABb0KFEfmRz+Z_-7GygTL12E5Y254dvoUfWe4uSv9-wOx+Cs8w@xxxxxxxxxxxxxx
>
>
> >
> >> +           total_pages > p->arg.max_pages) {
> >> +               size_t n_too_much = total_pages - p->arg.max_pages;
> >> +               *end -= n_too_much * PAGE_SIZE;
> >> +               n_pages -= n_too_much;
> >> +               ret = -ENOSPC;
> >> +       }
> >> +
> >> +       if (!pagemap_scan_push_range(categories, p, addr, *end)) {
> >> +               *end = addr;
> >> +               n_pages = 0;
> >> +               ret = -ENOSPC;
> >> +       }
> >> +
> >> +       p->found_pages += n_pages;
> >> +       if (ret)
> >> +               p->walk_end_addr = *end;
> >> +
> >> +       return ret;
> >> +}
> >> +
> >
> > ...
> >
> >> +static long do_pagemap_scan(struct mm_struct *mm, unsigned long uarg)
> >> +{
> >> +       struct mmu_notifier_range range;
> >> +       struct pagemap_scan_private p;
> >> +       unsigned long walk_start;
> >> +       size_t n_ranges_out = 0;
> >> +       int ret;
> >> +
> >> +       memset(&p, 0, sizeof(p));
> >> +       ret = pagemap_scan_get_args(&p.arg, uarg);
> >> +       if (ret)
> >> +               return ret;
> >> +
> >> +       p.masks_of_interest = MASKS_OF_INTEREST(p.arg);
> >> +       ret = pagemap_scan_init_bounce_buffer(&p);
> >> +       if (ret)
> >> +               return ret;
> >> +
> >> +       /* Protection change for the range is going to happen. */
> >> +       if (p.arg.flags & PM_SCAN_WP_MATCHING) {
> >> +               mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_VMA, 0,
> >> +                                       mm, p.arg.start, p.arg.end);
> >> +               mmu_notifier_invalidate_range_start(&range);
> >> +       }
> >> +
> >> +       walk_start = p.arg.start;
> >> +       for (; walk_start < p.arg.end; walk_start = p.arg.walk_end) {
> >> +               int n_out;
> >> +
> >> +               if (fatal_signal_pending(current)) {
> >> +                       ret = -EINTR;
> >> +                       break;
> >> +               }
> >> +
> >> +               ret = mmap_read_lock_killable(mm);
> >> +               if (ret)
> >> +                       break;
> >> +               ret = walk_page_range(mm, walk_start, p.arg.end,
> >> +                                     &pagemap_scan_ops, &p);
> >> +               mmap_read_unlock(mm);
> >> +
> >> +               n_out = pagemap_scan_flush_buffer(&p);
> >> +               if (n_out < 0)
> >> +                       ret = n_out;
> >> +               else
> >> +                       n_ranges_out += n_out;
> >> +
> >> +               if (ret != -ENOSPC || p.arg.vec_len - 1 == 0 ||
> >> +                   p.found_pages == p.arg.max_pages) {
> >> +                       p.walk_end_addr = p.arg.end;
> >
> > You should not change p.walk_end_addr If ret is ENOSPC. Pls add a test
> > case to check this.
> Yeah, I'm not setting walk_end_addr if ret is ENOSPC.
>
> I'm setting walk_end_addr only when ret = 0. I'd added this as a result of
> a test case in my local test application. I can look at adding some tests
> in pagemap_ioctl.c kselftest as well.

I am not sure that I understand what you mean here. ENOSPC can be returned
when the vec array is full and in this case, walk_end_addr should be
the address when it stops scanning.

>
> >
> >> +                       break;
> >> +               }
> >> +       }
> >> +
> >> +       if (p.cur_buf.start != p.cur_buf.end) {
> >> +               if (copy_to_user(p.vec_out, &p.cur_buf, sizeof(p.cur_buf)))
> >> +                       ret = -EFAULT;
> >> +               else
> >> +                       ++n_ranges_out;
> >> +       }
> >> +
> >> +       /* ENOSPC signifies early stop (buffer full) from the walk. */
> >> +       if (!ret || ret == -ENOSPC)
> >> +               ret = n_ranges_out;
> >> +
> >> +       p.arg.walk_end = p.walk_end_addr ? p.walk_end_addr : walk_start;
> >> +       if (pagemap_scan_writeback_args(&p.arg, uarg))
> >> +               ret = -EFAULT;
> >> +
> >> +       if (p.arg.flags & PM_SCAN_WP_MATCHING)
> >> +               mmu_notifier_invalidate_range_end(&range);
> >> +
> >> +       kfree(p.vec_buf);
> >> +       return ret;
> >> +}
> >
> > Thanks,
> > Andrei
>
> --
> BR,
> Muhammad Usama Anjum