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

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Message-ID: <ZNya0c7zRmQ/HPMl@qmqm.qmqm.pl>
Date:   Wed, 16 Aug 2023 11:45:53 +0200
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc:     Peter Xu <peterx@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michał Mirosław <emmir@...gle.com>,
        Andrei Vagin <avagin@...il.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Yun Zhou <yun.zhou@...driver.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alex Sierra <alex.sierra@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>, kernel@...labora.com,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v30 2/6] fs/proc/task_mmu: Implement IOCTL to get and
 optionally clear info about PTEs

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