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From: Bill O'Donnell <billodo@redhat.com>
To: Eric Sandeen <sandeen@sandeen.net>
Cc: "Darrick J. Wong" <djwong@kernel.org>,
	Bill O'Donnell <bodonnel@redhat.com>,
	linux-xfs@vger.kernel.org
Subject: Re: [PATCH] xfs: dax: facilitate EXPERIMENTAL warning for dax=inode case
Date: Fri, 27 Aug 2021 09:25:09 -0500	[thread overview]
Message-ID: <20210827142509.bjovj2l75xjoqd6w@redhat.com> (raw)
In-Reply-To: <0876d0d8-557a-db32-f2c3-9d976cab6fad@sandeen.net>

On Fri, Aug 27, 2021 at 09:18:32AM -0500, Eric Sandeen wrote:
> On 8/27/21 9:03 AM, Bill O'Donnell wrote:
> > On Thu, Aug 26, 2021 at 06:43:44PM -0500, Eric Sandeen wrote:
> > > On 8/26/21 5:08 PM, Bill O'Donnell wrote:
> > > > On Thu, Aug 26, 2021 at 01:16:22PM -0500, Eric Sandeen wrote:
> > > > > 
> > > > > On 8/26/21 1:09 PM, Darrick J. Wong wrote:
> > > > > > On Thu, Aug 26, 2021 at 12:30:12PM -0500, Bill O'Donnell wrote:
> > > > > 
> > > > > > > @@ -1584,7 +1586,7 @@ xfs_fs_fill_super(
> > > > > > >     	if (xfs_has_crc(mp))
> > > > > > >     		sb->s_flags |= SB_I_VERSION;
> > > > > > > -	if (xfs_has_dax_always(mp)) {
> > > > > > > +	if (xfs_has_dax_always(mp) || xfs_has_dax_inode(mp)) {
> > > > > > 
> > > > > > Er... can't this be done without burning another feature bit by:
> > > > > > 
> > > > > > 	if (xfs_has_dax_always(mp) || (!xfs_has_dax_always(mp) &&
> > > > > > 				       !xfs_has_dax_never(mp))) {
> > > > > > 		...
> > > > > > 		xfs_warn(mp, "DAX IS EXPERIMENTAL");
> > > > > > 	}
> > > > > 
> > > > > changing this conditional in this way will also fail dax=inode mounts on
> > > > > reflink-capable (i.e. default) filesystems, no?
> > > > 
> > > > Correct. My original patch tests fine, and still handles the reflink and dax
> > > > incompatibility. The new suggested logic is problematic.
> > > > -Bill
> > > 
> > > I think that both your proposed patch and Darrick's suggestion have this problem.
> > > 
> > > "mount -o dax=inode" makes your new xfs_has_dax_inode(mp) true, and in that
> > > conditional, if the filesystem has reflink enabled, mount fails:
> > > 
> > > # mkfs.xfs -f /dev/pmem0p1
> > > meta-data=/dev/pmem0p1           isize=512    agcount=4, agsize=4194304 blks
> > >           =                       sectsz=4096  attr=2, projid32bit=1
> > >           =                       crc=1        finobt=1, sparse=1, rmapbt=0
> > >           =                       reflink=1    bigtime=0 inobtcount=0
> > > data     =                       bsize=4096   blocks=16777216, imaxpct=25
> > >           =                       sunit=0      swidth=0 blks
> > > naming   =version 2              bsize=4096   ascii-ci=0, ftype=1
> > > log      =internal log           bsize=4096   blocks=8192, version=2
> > >           =                       sectsz=4096  sunit=1 blks, lazy-count=1
> > > realtime =none                   extsz=4096   blocks=0, rtextents=0
> > > 
> > > # mount -o dax=inode /dev/pmem0p1 /mnt/test
> > > mount: wrong fs type, bad option, bad superblock on /dev/pmem0p1,
> > >         missing codepage or helper program, or other error
> > > 
> > >         In some cases useful info is found in syslog - try
> > >         dmesg | tail or so.
> > > 
> > > # dmesg | tail -n 2
> > > [  192.691733] XFS (pmem0p1): DAX enabled. Warning: EXPERIMENTAL, use at your own risk
> > > [  192.700300] XFS (pmem0p1): DAX and reflink cannot be used together!
> > > 
> > 
> > So, the "DAX enabled" is a misnomer in this case. However the incompatibility of DAX and reflink is
> > reflected in the next message, and indeed the mount fails. Is it now a matter of fixing
> > the message output so as not to indicate "DAX enabled..."?
> 
> The mount should not fail, and it does not fail prior to your change.
> 
> In the past, we did not allow any mixing of a reflink-capable
> filesystem with dax in any way.  Now, with per-inode dax, dax-enabled inodes and
> reflink-enabled inodes can exist on the same filesystem, you just cannot have an
> inode which is both dax-enabled and reflinked at the same time.

Ah. I missed that nuance. I had thought the incompatibility was
absolute. :/

The manpage for mkfs.xfs may need updating for the inode mode
(unless mine is old):
----------------snip------------------
"Note:  the  filesystem DAX mount option ( -o dax ) is incom‐
patible  with  reflink-enabled  XFS  filesystems.   To   use
filesystem  DAX with XFS, specify the -m reflink=0 option to
mkfs.xfs to disable the reflink feature."
-------------------------------------
Thanks-
Bill


  reply	other threads:[~2021-08-27 14:25 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-26 17:30 Bill O'Donnell
2021-08-26 18:09 ` Darrick J. Wong
2021-08-26 18:14   ` Bill O'Donnell
2021-08-26 18:16   ` Eric Sandeen
2021-08-26 22:08     ` Bill O'Donnell
2021-08-26 23:43       ` Eric Sandeen
2021-08-27 14:03         ` Bill O'Donnell
2021-08-27 14:18           ` Eric Sandeen
2021-08-27 14:25             ` Bill O'Donnell [this message]
2021-08-27 15:35               ` Eric Sandeen
2021-08-27 15:41                 ` Bill O'Donnell
2021-08-30 15:55   ` Bill O'Donnell

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