Re: [PATCH 2/2] xfs_db: use directio for device access

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

On Wed, Jan 17, 2024 at 10:30:23AM -0800, Christoph Hellwig wrote:
> On Mon, Sep 25, 2023 at 02:59:16PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> > 
> > XFS and tools (mkfs, copy, repair) don't generally rely on the block
> > device page cache, preferring instead to use directio.  For whatever
> > reason, the debugger was never made to do this, but let's do that now.
> > 
> > This should eliminate the weird fstests failures resulting from
> > udev/blkid pinning a cache page while the unmounting filesystem writes
> > to the superblock such that xfs_db finds the stale pagecache instead of
> > the post-unmount superblock.
> 
> After some debugging I found out that this breaks a bunch of tests
> (at least xfs/002 xfs/070 xfs/424 in the quick group) on 4k device
> because xfs_db tries some unaligned reads.
> 
> For xfs/002 that is the libxfs_buf_read in __set_cur, when setting the
> type to data, but I haven't looked at the other test in detail.

Hmm.  Perhaps the userspace buftarg setup should go find the physical
sector size of the device?  That "bb_count = 1" in set_iocur_type looks
a bit smelly.

> Should I look into finding all these assumptions in xfs_db, or
> just make the direct I/O enablement conditional n a 612 byte sector
> size?

Let me go run a lbasize=4k fstests run overnight and see what happens.
IIRC zorro told me last year that it wasn't pretty.

--D