Re: [PATCH v30 2/6] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

[Michał Mirosław]

On Wed, Aug 16, 2023 at 11:59:21AM +0500, Muhammad Usama Anjum wrote:
> The PAGEMAP_SCAN IOCTL on the pagemap file can be used to get or optionally
> clear the info about page table entries.
[...]
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
[...]
> +static long do_pagemap_scan(struct mm_struct *mm, unsigned long uarg)
> +{
[...]
> +	for (walk_start = p.arg.start; walk_start < p.arg.end;
> +			walk_start = p.arg.walk_end) {
> +		long 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;
> +
> +		p.arg.walk_end = p.walk_end_addr ? p.walk_end_addr : p.arg.end;

I think p.walk_end_addr can be removed and replaced by `p.arg.walk_end`
directly in the walk functions. If we don't set walk_end_addr we'll also
return 0 so the check below will match. Might be good to add this as
a comment.

> +		if (ret != -ENOSPC)
> +			break;
> +
> +		if (p.arg.vec_len == 0 || p.found_pages == p.arg.max_pages)
> +			break;
> +	}
> +
> +	/* ENOSPC signifies early stop (buffer full) from the walk. */
> +	if (!ret || ret == -ENOSPC)
> +		ret = n_ranges_out;
> +
> +	p.arg.walk_end = p.arg.walk_end ? p.arg.walk_end : walk_start;

When the walk is finished, with ret == 0, the walk_start will point to
the beginning, not the end of the range. So:

if (!walk_end) walk_end = p.arg.end;

Other than that, the patch looks complete now. Thanks for all your work!

Best Regards
Michał Mirosław