Re: [PATCH v26 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

On Thu, Jul 27, 2023 at 2:37 AM Muhammad Usama Anjum usama.anjum@collabora.com wrote:

<snip>

+static long do_pagemap_scan(struct mm_struct *mm, unsigned long uarg) +{

•   unsigned long walk_start, walk_end;
  

•   struct mmu_notifier_range range;
  

•   struct pagemap_scan_private p;
  

•   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;
  

•   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 = walk_end = p.arg.start;
  

•   for (; walk_end != p.arg.end; walk_start = walk_end) {
  

•           int n_out;
  

•           walk_end = min_t(unsigned long,
  

•                            (walk_start + PAGEMAP_WALK_SIZE) & PAGEMAP_WALK_MASK,
  

•                            p.arg.end);
  

This approach has performance implications. The basic program that scans its address space takes around 20-30 seconds, but it has just a few small mappings. The first optimization that comes to mind is to remove the PAGEMAP_WALK_SIZE limit and instead halt walk_page_range when the bounce buffer is full. After draining the buffer, the walk_page_range function can be restarted.

The test program and perf data can be found here: https://gist.github.com/avagin/c5a22f3c78f8cb34281602dfe9c43d10

•           ret = mmap_read_lock_killable(mm);
  

•           if (ret)
  

•                   break;
  

•           ret = walk_page_range(mm, walk_start, walk_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)
  

•                   break;
  

•   }
  

Thanks, Andrei