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

On 8/9/23 12:55 AM, Andrei Vagin wrote:

On Tue, Aug 8, 2023 at 12:35 PM Muhammad Usama Anjum usama.anjum@collabora.com wrote:

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On 8/9/23 12:21 AM, Andrei Vagin wrote:

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On Tue, Aug 8, 2023 at 3:43 AM Muhammad Usama Anjum usama.anjum@collabora.com wrote:

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+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+C...

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•       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;
  

+}

...

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+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.

I'll copy a test case in kselftest to prove or dis-prove the correctness of walk_end address.

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...

•                   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