Re: [bug report] xfs/802 failure due to mssing fstype report by lsblk

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Feb 09, 2026 at 07:54:38AM +0000, Shinichiro Kawasaki wrote:
> On Feb 09, 2026 / 07:28, hch wrote:
> > On Sun, Feb 08, 2026 at 10:07:16PM -0800, Darrick J. Wong wrote:
> > > Waitaminute, how can you even format xfs on nullblk to run fstests?
> > > Isn't that the bdev that silently discards everything written to it, and
> > > returns zero on reads??
> > 
> > nullblk can be used with or without a backing store.  In the former
> > case it will not always return zeroes on reads obviously.
> 
> Yes, null_blk has the "memory_backed" parameter. When 1 is set to this, data
> written to the null_blk device is kept and read back. I create two 8GiB null_blk
> devices enabling this memory_backed option, and use them as TEST_DEV and
> SCRATCH_DEV for the regular xfs test runs.

Huh, ok.  Just out of curiosity, does blkid (in cache mode) /ever/ see
the xfs filesystem?  I'm wondering if there's a race, a slow utility, or
if whatever builds the blkid cache sees that it's nullblk and ignores
it?

--D