Date:   Tue, 10 May 2022 09:43:28 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org
Cc:     mhocko@...e.com, rientjes@...gle.com, willy@...radead.org,
        hannes@...xchg.org, guro@...com, minchan@...nel.org,
        kirill@...temov.name, aarcange@...hat.com, brauner@...nel.org,
        hch@...radead.org, oleg@...hat.com, david@...hat.com,
        jannh@...gle.com, shakeelb@...gle.com, peterx@...hat.com,
        jhubbard@...dia.com, shuah@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org, kernel-team@...roid.com,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH 1/3] selftests: vm: add process_mrelease tests

On 5/9/22 9:00 PM, Suren Baghdasaryan wrote:
> Introduce process_mrelease syscall sanity tests. They include tests of
> invalid pidfd and flags inputs, attempting to call process_mrelease
> with a live process and a valid usage of process_mrelease. Because
> process_mrelease has to be used against a process with a pending SIGKILL,
> it's possible that the process exits before process_mrelease gets called.
> In such cases we retry the test with a victim that allocates twice more
> memory up to 1GB. This would require the victim process to spend more
> time during exit and process_mrelease has a better chance of catching
> the process before it exits.
> 

+1 on Mike's comments on improving the change log. List what is getting
tested as opposed to describing the test code.

> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> ---
>   tools/testing/selftests/vm/Makefile        |   1 +
>   tools/testing/selftests/vm/mrelease_test.c | 176 +++++++++++++++++++++
>   tools/testing/selftests/vm/run_vmtests.sh  |  16 ++
>   3 files changed, 193 insertions(+)
>   create mode 100644 tools/testing/selftests/vm/mrelease_test.c

Please update .gitignore with the new executable.

> 
> diff --git a/tools/testing/selftests/vm/Makefile b/tools/testing/selftests/vm/Makefile
> index 04a49e876a46..733fccbff0ef 100644
> --- a/tools/testing/selftests/vm/Makefile
> +++ b/tools/testing/selftests/vm/Makefile
> @@ -43,6 +43,7 @@ TEST_GEN_FILES += map_populate
>   TEST_GEN_FILES += memfd_secret
>   TEST_GEN_FILES += mlock-random-test
>   TEST_GEN_FILES += mlock2-tests
> +TEST_GEN_FILES += mrelease_test
>   TEST_GEN_FILES += mremap_dontunmap
>   TEST_GEN_FILES += mremap_test
>   TEST_GEN_FILES += on-fault-limit
> diff --git a/tools/testing/selftests/vm/mrelease_test.c b/tools/testing/selftests/vm/mrelease_test.c
> new file mode 100644
> index 000000000000..a61061bf8433
> --- /dev/null
> +++ b/tools/testing/selftests/vm/mrelease_test.c
> @@ -0,0 +1,176 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright 2022 Google LLC
> + */
> +#define _GNU_SOURCE
> +#include <errno.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <sys/wait.h>
> +#include <unistd.h>
> +
> +#include "util.h"
> +
> +static inline int pidfd_open(pid_t pid, unsigned int flags)
> +{
> +#ifdef __NR_pidfd_open
> +	return syscall(__NR_pidfd_open, pid, flags);
> +#else
> +	errno = ENOSYS;

This isn't an error - this would be skip because this syscall
isn't supported.

> +	return -1;
> +#endif

Key off of syscall return instead of these ifdefs - same comment
on all of the ifdefs

> +}
> +

I am not seeing any reason for breaking this code up have a separate
routine for pidfd_open().

> +static inline int process_mrelease(int pidfd, unsigned int flags)
> +{
> +#ifdef __NR_process_mrelease
> +	return syscall(__NR_process_mrelease, pidfd, flags);
> +#else
> +	errno = ENOSYS;
> +	return -1;
> +#endif> +}
> +

Same comments on ifdefs and skips here as well.

> +static void write_fault_pages(char *addr, unsigned long nr_pages)
> +{
> +	unsigned long i;
> +
> +	for (i = 0; i < nr_pages; i++)
> +		*((unsigned long *)(addr + (i * PAGE_SIZE))) = i;
> +}
> +
> +static int alloc_noexit(unsigned long nr_pages, int pipefd)
> +{
> +	int ppid = getppid();
> +	void *buf;
> +
> +	buf = mmap(NULL, nr_pages * PAGE_SIZE, PROT_READ | PROT_WRITE,
> +		   MAP_PRIVATE | MAP_ANON, 0, 0);
> +	if (buf == MAP_FAILED) {
> +		perror("mmap");

A bit more descriptive message what the test would do will be helpful.
Also consider if this should be a skip or fail for the test.

> +		return 1;
> +	}
> +
> +	write_fault_pages((char *)buf, nr_pages);
> +
> +	/* Signal the parent that the child is ready */
> +	if (write(pipefd, "", 1) < 0) {
> +		perror("write");
> +		return 1;
> +	}
> +
> +	/* Wait to be killed (when reparenting happens) */
> +	while (getppid() == ppid)
> +		sleep(1);
> +

What happens if reparenting doesn't happen? Will this loop for ever?
This test could hang?

> +	munmap(buf, nr_pages * PAGE_SIZE);
> +
> +	return 0;
> +}
> +
> +
> +#define MB(x) (x << 20)
> +#define MAX_SIZE_MB 1024
> +
> +int main(void)
> +{
> +	int res;
> +	int pipefd[2], pidfd;
> +	pid_t pid;
> +	char byte;
> +	size_t size;
> +	int negative_tests_done = 0;
> +
> +	/* Test a wrong pidfd */
> +	if (!process_mrelease(-1, 0) || errno != EBADF) {
> +		perror("process_mrelease with wring pidfd");

Incorrect spelling "wring/wrong"

> +		exit(1);
> +	}
> +
> +	/*
> +	 * Start the test with 1MB allocation and double every time
> +	 * process_mrelease fails
> +	 */
> +	for (size = 1; size <= MAX_SIZE_MB; size *= 2) {
> +		/*
> +		 * Pipe for the child to signal when it's done allocating
> +		 * memory
> +		 */
> +		if (pipe(pipefd)) {
> +			perror("pipe");
> +			exit(1);
> +		}
> +		pid = fork();
> +		if (pid < 0) {
> +			perror("fork");

Close the pipe?

> +			exit(1);
> +		}
> +
> +		if (pid == 0) {
> +			close(pipefd[0]);
> +			res = alloc_noexit(MB(size) / PAGE_SIZE, pipefd[1]);
> +			close(pipefd[1]);
> +			exit(res);
> +		}
> +
> +		close(pipefd[1]);
> +		/* Block until the child is ready */
> +		res = read(pipefd[0], &byte, 1);
> +		close(pipefd[0]);
> +		if (res < 0) {
> +			perror("read");
> +			exit(1);
> +		}
> +
> +		pidfd = pidfd_open(pid, 0);
> +		if (pidfd < 0) {
> +			perror("pidfd_open");
> +			exit(1);
> +		}
> +

The code is very hard to read. Add comments to indicate parent and child
paths clearly so reviewers can follow the logic and be able to do effective
review.

> +		/* Run negative tests which require a valid child only once */
> +		if (!negative_tests_done) {
> +			/* Test invalid flags */
> +			if (!process_mrelease(pidfd, (unsigned int)-1) ||
> +			    errno != EINVAL) {
> +				perror("process_mrelease with wrong flags");
> +				exit(1);

So is this an expected fail or a test fail?

> +			}
> +			/* Test reapling while process is still alive */
> +			if (!process_mrelease(pidfd, 0) ||
> +			    errno != EINVAL) {
> +				perror("process_mrelease on a live process");

So is this an expected fail or a test fail?

> +				exit(1);
> +			}
> +			negative_tests_done = 1;
> +		}

Now the above negative_tests_done block could be in a separate function ---
All the others aren't really needed. It will be good for abstraction and
readability.

> +
> +		if (kill(pid, SIGKILL)) {
> +			perror("kill");

Include test results in the change log - so we can see the test report.

> +			exit(1);
> +		}
> +
> +		if (!process_mrelease(pidfd, 0)) {
> +			/* Terminate the test once process_mrelease succeeds */
> +			return 0;
> +		}
> +
> +		/*
> +		 * Ignore the failure if the child exited before mrelease got
> +		 * called, increase allocation size and retry the test
> +		 */

Add more info. on why allocating more memory helps.

> +		if (errno != ESRCH) {
> +			perror("process_mrelease");
> +			exit(1);
> +		}
> +
> +		if (waitpid(pid, NULL, 0) < 0) {
> +			perror("waitpid");
> +			exit(1);
> +		}
> +		close(pidfd);
> +	}
> +
> +	printf("All process_mrelease attempts failed!\n");
> +	exit(1);
> +}
> diff --git a/tools/testing/selftests/vm/run_vmtests.sh b/tools/testing/selftests/vm/run_vmtests.sh
> index 352ba00cf26b..1986162fea39 100755
> --- a/tools/testing/selftests/vm/run_vmtests.sh
> +++ b/tools/testing/selftests/vm/run_vmtests.sh
> @@ -287,6 +287,22 @@ else
>   	echo "[PASS]"
>   fi
>   
> +echo "---------------------"
> +echo "running mrelease_test"
> +echo "---------------------"
> +./mrelease_test
> +ret_val=$?
> +
> +if [ $ret_val -eq 0 ]; then
> +	echo "[PASS]"
> +elif [ $ret_val -eq $ksft_skip ]; then
> +	 echo "[SKIP]"
> +	 exitcode=$ksft_skip
> +else
> +	echo "[FAIL]"
> +	exitcode=1
> +fi
> +
>   echo "-------------------"
>   echo "running mremap_test"
>   echo "-------------------"
> 

In general, the code flow is hard to read to make sure resources
are released e.g: pipefd in all the error paths. The code is broken
up into smaller chunks where it isn't needed in some cases and left
as a large block when it could benefit from abstraction e.g: negative
test block.

Please make changes and send v2.

thanks,
-- Shuah

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