_require_sparse_files: rewrite as a direct test instead of a black list

_require_sparse_files was implemented as a list of filesystems known not to
support sparse files, and therefore it missed some cases.

However, if sparse files do not work as expected during a test, the risk
is that the test will write out to the disk all the zeros that would
normally be unwritten. This amounts to at least 4 TB for the generic/129
test, and therefore there is a significant media wear-out concern here.

Adding more filesystems to the list of exclusions would not scale and
would not work anyway because CIFS backed by SAMBA is safe, while CIFS
backed by Windows Server 2022 is not (because the specific write
patterns found in generic/014 and generic/129 cause it to ignore the
otherwise-supported request to make a file sparse).

Mitigate this risk by rewriting the check as a small-scale test that
reliably triggers Windows misbehavior. The black list becomes unneeded
because the same test creates and detects non-sparse files on exfat and

Signed-off-by: Alexander Patrakov <patrakov@gmail.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>
1 file changed