Subject: Re: [PATCH] selftests/mount_setattr: Fix failures on 64K PAGE_SIZE kernels

From: Ritesh Harjani (IBM) <[email-masked]>
Date: Sat, 16 Nov 2024 17:12:34 +0530

Michael Ellerman <[email-masked]> writes:

> Currently the mount_setattr_test fails on machines with a 64K PAGE_SIZE,
> with errors such as:
>
>   #  RUN           mount_setattr_idmapped.invalid_fd_negative ...
>   mkfs.ext4: No space left on device while writing out and closing file system
>   # mount_setattr_test.c:1055:invalid_fd_negative:Expected system("mkfs.ext4 -q /mnt/C/ext4.img") (256) == 0 (0)
>   # invalid_fd_negative: Test terminated by assertion
>   #          FAIL  mount_setattr_idmapped.invalid_fd_negative
>   not ok 12 mount_setattr_idmapped.invalid_fd_negative
>
> The code creates a 100,000 byte tmpfs:
>
> 	ASSERT_EQ(mount("testing", "/mnt", "tmpfs", MS_NOATIME | MS_NODEV,
> 			"size=100000,mode=700"), 0);
>
> And then a little later creates a 2MB ext4 filesystem in that tmpfs:
>
> 	ASSERT_EQ(ftruncate(img_fd, 1024 * 2048), 0);
> 	ASSERT_EQ(system("mkfs.ext4 -q /mnt/C/ext4.img"), 0);
>
> At first glance it seems like that should never work, after all 2MB is
> larger than 100,000 bytes. However the filesystem image doesn't actually
> occupy 2MB on "disk" (actually RAM, due to tmpfs). On 4K kernels the
> ext4.img uses ~84KB of actual space (according to du), which just fits.
>
> However on 64K PAGE_SIZE kernels the ext4.img takes at least 256KB,
> which is too large to fit in the tmpfs, hence the errors.

Even though ext4 will use by default 4k blocksize on both kernels but
the backing file here belongs to tmpfs (RAM) which operates at 64k page.
Hence the difference in the size of the image between 4k and 64k kernels
looks expected.

# 64k kernel
du -sh /run/ext4
256K    /run/ext4

# 4k kernel
du -sh /run/ext4
84K     /run/ext4

>
> It seems fraught to rely on the ext4.img taking less space on disk than
> the allocated size, so instead create the tmpfs with a size of 2MB. With
> that all 21 tests pass on 64K PAGE_SIZE kernels.

That looks like the right thing to do here. 

Please feel free to add - 

Reviewed-by: Ritesh Harjani (IBM) <[email-masked]>

>
> Fixes: 01eadc8dd96d ("tests: add mount_setattr() selftests")
> Signed-off-by: Michael Ellerman <[email-masked]>
> ---
>  tools/testing/selftests/mount_setattr/mount_setattr_test.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/mount_setattr/mount_setattr_test.c b/tools/testing/selftests/mount_setattr/mount_setattr_test.c
> index 68801e1a9ec2..70f65eb320a7 100644
> --- a/tools/testing/selftests/mount_setattr/mount_setattr_test.c
> +++ b/tools/testing/selftests/mount_setattr/mount_setattr_test.c
> @@ -1026,7 +1026,7 @@ FIXTURE_SETUP(mount_setattr_idmapped)
>  			"size=100000,mode=700"), 0);
>  
>  	ASSERT_EQ(mount("testing", "/mnt", "tmpfs", MS_NOATIME | MS_NODEV,
> -			"size=100000,mode=700"), 0);
> +			"size=2m,mode=700"), 0);
>  
>  	ASSERT_EQ(mkdir("/mnt/A", 0777), 0);
>  
> -- 
> 2.47.0