On Thu, Jul 27, 2023 at 2:37 AM Muhammad Usama Anjum
<usama.anjum@xxxxxxxxxxxxx> 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
