Re: [PATCH v2 4/4] tests/qtest/migration-test: Add a postcopy memfile test

[Peter Xu <peterx@redhat.com>]

On Thu, May 30, 2024 at 07:54:07PM +1000, Nicholas Piggin wrote:
> Postcopy requires userfaultfd support, which requires tmpfs if a memory
> file is used.
> 
> This adds back support for /dev/shm memory files, but adds preallocation
> to skip environments where that mount is limited in size.
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

Thanks for doing this regardless.

> ---
>  tests/qtest/migration-test.c | 77 ++++++++++++++++++++++++++++++++----
>  1 file changed, 69 insertions(+), 8 deletions(-)
> 
> diff --git a/tests/qtest/migration-test.c b/tests/qtest/migration-test.c
> index 86eace354e..5078033ded 100644
> --- a/tests/qtest/migration-test.c
> +++ b/tests/qtest/migration-test.c
> @@ -11,6 +11,7 @@
>   */
>  
>  #include "qemu/osdep.h"
> +#include "qemu/cutils.h"
>  
>  #include "libqtest.h"
>  #include "qapi/qmp/qdict.h"
> @@ -553,6 +554,7 @@ typedef struct {
>       */
>      bool hide_stderr;
>      bool use_memfile;
> +    bool use_shm_memfile;

Nitpick: when having both, it's slightly confusing on the name, e.g. not
clear whether use_memfile needs to be set to true too if use_shm_memfile=true.

Maybe "use_tmpfs_memfile" and "use_shm_memfile"?

>      /* only launch the target process */
>      bool only_target;
>      /* Use dirty ring if true; dirty logging otherwise */
> @@ -739,7 +741,62 @@ static int test_migrate_start(QTestState **from, QTestState **to,
>          ignore_stderr = "";
>      }
>  
> -    if (args->use_memfile) {
> +    if (!qtest_has_machine(machine_alias)) {
> +        g_autofree char *msg = g_strdup_printf("machine %s not supported",
> +                                               machine_alias);
> +        g_test_skip(msg);
> +        return -1;
> +    }
> +
> +    if (args->use_shm_memfile) {
> +#if defined(__NR_userfaultfd) && defined(__linux__)

IIUC we only need defined(__linux__) as there's nothing to do with uffd yet?

> +        int fd;
> +        uint64_t size;
> +
> +        if (getenv("GITLAB_CI")) {
> +            /*
> +             * Gitlab runners are limited to 64MB shm size and despite
> +             * pre-allocation there is concern that concurrent tests
> +             * could result in nondeterministic failures. Until all shm
> +             * usage in all CI tests is found to fail gracefully on
> +             * ENOSPC, it is safer to avoid large allocations for now.
> +             *
> +             * https://lore.kernel.org/qemu-devel/875xuwg4mx.fsf@suse.de/
> +             */
> +            g_test_skip("shm tests are not supported in Gitlab CI environment");
> +            return -1;
> +        }

I'm not sure whether this is Fabiano's intention.  I'm wondering whether we
can drop this and just let it still run there.

Other tests not detecting avaiablility of shmem looks like a separate issue
to be fixed to me, regardless of this.

My wild guess is since we're doing memory_size+64K below then if test won't
fail others won't either, as normally the shmem quota should normally be
power of 2 anyway.. then it should always fit another few MBs if this one.
While this test is ready to fail gracefully now.

> +
> +        if (!g_file_test("/dev/shm", G_FILE_TEST_IS_DIR)) {
> +            g_test_skip("/dev/shm does not exist or is not a directory");
> +            return -1;
> +        }
> +
> +        /*
> +         * Pre-create and allocate the file here, because /dev/shm/
> +         * is known to be limited in size in some places (e.g., Gitlab CI).
> +         */
> +        memfile_path = g_strdup_printf("/dev/shm/qemu-%d", getpid());
> +        fd = open(memfile_path, O_WRONLY | O_CREAT | O_EXCL, S_IRUSR | S_IWUSR);
> +        if (fd == -1) {
> +            g_test_skip("/dev/shm file could not be created");
> +            return -1;
> +        }
> +
> +        g_assert(qemu_strtosz(memory_size, NULL, &size) == 0);
> +        size += 64*1024; /* QEMU may map a bit more memory for a guard page */
> +
> +        if (fallocate(fd, 0, 0, size) == -1) {
> +            unlink(memfile_path);
> +            perror("could not alloc"); exit(1);
> +            g_test_skip("Could not allocate machine memory in /dev/shm");
> +            return -1;
> +        }
> +        close(fd);
> +#else
> +        g_test_skip("userfaultfd is not supported");

"/dev/shm not available" instead?

> +#endif
> +    } else if (args->use_memfile) {
>          memfile_path = g_strdup_printf("/%s/qemu-%d", tmpfs, getpid());
>          memfile_opts = g_strdup_printf(
>              "-object memory-backend-file,id=mem0,size=%s"
> @@ -751,12 +808,6 @@ static int test_migrate_start(QTestState **from, QTestState **to,
>          kvm_opts = ",dirty-ring-size=4096";
>      }
>  
> -    if (!qtest_has_machine(machine_alias)) {
> -        g_autofree char *msg = g_strdup_printf("machine %s not supported", machine_alias);
> -        g_test_skip(msg);
> -        return -1;
> -    }
> -
>      machine = resolve_machine_version(machine_alias, QEMU_ENV_SRC,
>                                        QEMU_ENV_DST);
>  
> @@ -807,7 +858,7 @@ static int test_migrate_start(QTestState **from, QTestState **to,
>       * Remove shmem file immediately to avoid memory leak in test failed case.
>       * It's valid because QEMU has already opened this file
>       */
> -    if (args->use_memfile) {
> +    if (args->use_memfile || args->use_shm_memfile) {
>          unlink(memfile_path);
>      }
>  
> @@ -1275,6 +1326,15 @@ static void test_postcopy(void)
>      test_postcopy_common(&args);
>  }
>  
> +static void test_postcopy_memfile(void)
> +{

IMHO the defined(__NR_userfaultfd) should be here to guard if needed.

Or rather, we don't need to care about uffd yet? As what we already do with
test_postcopy().

I'm guessing the test just doesn't run on !linux, while compilation always
works with/without that.

Thanks,

> +    MigrateCommon args = {
> +        .start.use_shm_memfile = true,
> +    };
> +
> +    test_postcopy_common(&args);
> +}
> +
>  static void test_postcopy_suspend(void)
>  {
>      MigrateCommon args = {
> @@ -3441,6 +3501,7 @@ int main(int argc, char **argv)
>  
>      if (has_uffd) {
>          migration_test_add("/migration/postcopy/plain", test_postcopy);
> +        migration_test_add("/migration/postcopy/memfile", test_postcopy_memfile);
>          migration_test_add("/migration/postcopy/recovery/plain",
>                             test_postcopy_recovery);
>          migration_test_add("/migration/postcopy/preempt/plain",
> -- 
> 2.43.0
> 

-- 
Peter Xu