tree e09d23f7f9ad28c1945dd1b92e6d8dad38d0a541
parent d27e715c3081306e1b210e64d21775457c9f087a
author Darrick J. Wong <djwong@kernel.org> 1713222448 -0700
committer Darrick J. Wong <djwong@kernel.org> 1713387982 -0700

mkfs: fix log sunit rounding when external logs are in use

Due to my heinous nature, I set up an external log device with 4k LBAs
using this command:

# losetup -b 4096 -o 4096 --sizelimit $(( (128 * 1048576) - 4096 )) -f /dev/sdb
# blockdev --getsize64 /dev/loop0
134213632

This creates a log device that is slightly smaller than 128MB in size.
Next I ran generic/054, which sets the log sunit to 256k and fails:

# mkfs.xfs -f /dev/sda -l logdev=/dev/loop0,su=256k,version=2 -s size=4096
meta-data=/dev/sda               isize=512    agcount=4, agsize=72448 blks
         =                       sectsz=4096  attr=2, projid32bit=1
         =                       crc=1        finobt=1, sparse=1, rmapbt=1
         =                       reflink=1    bigtime=1 inobtcount=1 nrext64=1
         =                       metadir=0
data     =                       bsize=4096   blocks=289792, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0, ftype=1, parent=0
log      =/dev/loop0             bsize=4096   blocks=32768, version=2
         =                       sectsz=4096  sunit=64 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
         =                       rgcount=0    rgsize=0 blks
Discarding blocks...Done.
Discarding blocks...Done.
mkfs.xfs: libxfs_device_zero write failed: No space left on device

Notice that mkfs thinks it should format a 32768-fsblock external log,
but the log device itself is 32767 fsblocks.  Hence the write goes off
the end of the device and we get ENOSPC.

I tracked this behavior down to align_log_size in mkfs, which first
tries to round the log size up to a stripe boundary, then tries to round
it down.  Unfortunately, in the case of an external log we call the
function with XFS_MAX_LOG_BLOCKS without accounting for the possibility
that the log device might be smaller.

Correct the callsite and clean up the open-coded rounding.

Fixes: 8d1bff2be336 ("mkfs: reduce internal log size when log stripe units are in play")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>