linux-kernel - Re: [PATCH] selftests/mm: add fork CoW guard page test

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Message-ID: <qbzzv3lz62it6gitcttsxe2f2i7uq56pz6sfcbjtopatryeukn@hfy3d3l2uvoy>
Date: Thu, 5 Dec 2024 14:19:41 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Shuah Khan <shuah@...nel.org>,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH] selftests/mm: add fork CoW guard page test

* Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [241205 14:07]:
> When we fork anonymous pages, apply a guard page then remove it, the
> previous CoW mapping is cleared.
> 
> This might not be obvious to an outside observer without taking some time
> to think about how the overall process functions, so document that this is
> the case through a test, which also usefully asserts that the behaviour is
> as we expect.
> 
> This is grouped with other, more important, fork tests that ensure that
> guard pages are correctly propagated on fork.
> 
> Fix a typo in a nearby comment at the same time.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

Nice to see some more testing going in.

Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>

> ---
>  tools/testing/selftests/mm/guard-pages.c | 73 +++++++++++++++++++++++-
>  1 file changed, 72 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/mm/guard-pages.c b/tools/testing/selftests/mm/guard-pages.c
> index 7cdf815d0d63..d8f8dee9ebbd 100644
> --- a/tools/testing/selftests/mm/guard-pages.c
> +++ b/tools/testing/selftests/mm/guard-pages.c
> @@ -990,7 +990,7 @@ TEST_F(guard_pages, fork)
>  		   MAP_ANON | MAP_PRIVATE, -1, 0);
>  	ASSERT_NE(ptr, MAP_FAILED);
> 
> -	/* Establish guard apges in the first 5 pages. */
> +	/* Establish guard pages in the first 5 pages. */
>  	ASSERT_EQ(madvise(ptr, 5 * page_size, MADV_GUARD_INSTALL), 0);
> 
>  	pid = fork();
> @@ -1029,6 +1029,77 @@ TEST_F(guard_pages, fork)
>  	ASSERT_EQ(munmap(ptr, 10 * page_size), 0);
>  }
> 
> +/*
> + * Assert expected behaviour after we fork populated ranges of anonymous memory
> + * and then guard and unguard the range.
> + */
> +TEST_F(guard_pages, fork_cow)
> +{
> +	const unsigned long page_size = self->page_size;
> +	char *ptr;
> +	pid_t pid;
> +	int i;
> +
> +	/* Map 10 pages. */
> +	ptr = mmap(NULL, 10 * page_size, PROT_READ | PROT_WRITE,
> +		   MAP_ANON | MAP_PRIVATE, -1, 0);
> +	ASSERT_NE(ptr, MAP_FAILED);
> +
> +	/* Populate range. */
> +	for (i = 0; i < 10 * page_size; i++) {
> +		char chr = 'a' + (i % 26);
> +
> +		ptr[i] = chr;
> +	}
> +
> +	pid = fork();
> +	ASSERT_NE(pid, -1);
> +	if (!pid) {
> +		/* This is the child process now. */
> +
> +		/* Ensure the range is as expected. */
> +		for (i = 0; i < 10 * page_size; i++) {
> +			char expected = 'a' + (i % 26);
> +			char actual = ptr[i];
> +
> +			ASSERT_EQ(actual, expected);
> +		}
> +
> +		/* Establish guard pages across the whole range. */
> +		ASSERT_EQ(madvise(ptr, 10 * page_size, MADV_GUARD_INSTALL), 0);
> +		/* Remove it. */
> +		ASSERT_EQ(madvise(ptr, 10 * page_size, MADV_GUARD_REMOVE), 0);
> +
> +		/*
> +		 * By removing the guard pages, the page tables will be
> +		 * cleared. Assert that we are looking at the zero page now.
> +		 */
> +		for (i = 0; i < 10 * page_size; i++) {
> +			char actual = ptr[i];
> +
> +			ASSERT_EQ(actual, '\0');
> +		}
> +
> +		exit(0);
> +	}
> +
> +	/* Parent process. */
> +
> +	/* Parent simply waits on child. */
> +	waitpid(pid, NULL, 0);
> +
> +	/* Ensure the range is unchanged in parent anon range. */
> +	for (i = 0; i < 10 * page_size; i++) {
> +		char expected = 'a' + (i % 26);
> +		char actual = ptr[i];
> +
> +		ASSERT_EQ(actual, expected);
> +	}
> +
> +	/* Cleanup. */
> +	ASSERT_EQ(munmap(ptr, 10 * page_size), 0);
> +}
> +
>  /*
>   * Assert that forking a process with VMAs that do have VM_WIPEONFORK set
>   * behave as expected.
> --
> 2.47.1