linux-kernel - Re: [PATCH -next v1 3/3] kernel/events/uprobes: uprobe_write

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Message-ID: <20250310170320.GC26382@redhat.com>
Date: Mon, 10 Mar 2025 18:03:21 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-trace-kernel@...r.kernel.org,
	linux-perf-users@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Wilcox <willy@...radead.org>,
	Russell King <linux@...linux.org.uk>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	"Liang, Kan" <kan.liang@...ux.intel.com>,
	Tong Tiangen <tongtiangen@...wei.com>
Subject: Re: [PATCH -next v1 3/3] kernel/events/uprobes:
 uprobe_write_opcode() rewrite

On 03/04, David Hildenbrand wrote:
>
> uprobe_write_opcode() does some pretty low-level things that really, it
> shouldn't be doing:

Agreed. Thanks again for doing this.

David, as I said, I can't review. I don't understand this mm/folio magic
with or without your changes.

However. With your changes the code looks "better" and more understandable
to me. So I'd vote for your patches even if I can't ack them.

But I'd like to ask some stupid (no, really) questions.
__uprobe_write_opcode() does:

	/* We're done if we don't find an anonymous folio when unregistering. */
	if (!folio_test_anon(folio))
		return is_register ? -EFAULT : 0;

Yes, but we do not expect !folio_test_anon() if register == true, right?
See also below.

	/* Verify that the page content is still as expected. */
	if (verify_opcode(fw->page, opcode_vaddr, &opcode) <= 0) {
		set_pte_at(vma->vm_mm, vaddr, fw->ptep, fw->pte);
		return -EAGAIN;
	}

The caller, uprobe_write_opcode(), has already called verify_opcode(),
why do we need to re-check?

But whatever reason we have. Can we change uprobe_write_opcode() to
"delay" put_page() and instead of

	/* Walk the page tables again, to perform the actual update. */
	folio = folio_walk_start(&fw, vma, vaddr, 0);
	if (folio) {
		ret = __uprobe_write_opcode(vma, &fw, folio, opcode_vaddr,
					    opcode);
		folio_walk_end(&fw, vma);
	} else {
		ret = -EAGAIN;
	}

do something like

	/* Walk the page tables again, to perform the actual update. */
	ret = -EAGAIN;
	folio = folio_walk_start(&fw, vma, vaddr, 0);
	if (folio) {
		if (fw.page == page) {
			WARN_ON(is_register && !folio_test_anon(folio));
			ret = __uprobe_write_opcode(vma, &fw, folio, opcode_vaddr,
					            opcode);
		}
		folio_walk_end(&fw, vma);
	}

?

Once again, I am not trying to review. I am trying to understand the
basics of your code.

Thanks,

Oleg.