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

[Muhammad Usama Anjum]

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.

> 
>> +                       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