Re: [PATCH 0/4] xfs: online relabeling [RFC]

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Fri, Jun 10, 2016 at 08:19:36AM -0400, Brian Foster wrote:
> On Thu, Jun 09, 2016 at 11:51:12AM -0500, Eric Sandeen wrote:
> > 
> > 
> > On 6/9/16 11:36 AM, Eric Sandeen wrote:
> > > Ok, this more or less works; not really up to snuff
> > > for submission or merging, just sketching it out, but some
> > > questions first:
> > > 
> > > 1) Is there really any point to this? :) We did have one
> > >    request, and btrfs can do it ...
> > > 
> 
> Seems reasonable to me. Any details on the use case for the request?
> 
> > > 2) Is using m_growlock horrible?  growfs is the only other
> > >    thing that writes all supers, so I grabbed it.  We don't
> > >    want multiple relabels stepping on each other.
> > > 
> > > 3) Is there some way to actually force the primary to disk?
> > >    Right now the label change isn't actually visible on the
> > >    primary until unmount, which defeats the purpose.  I'm not
> > >    sure if there's a straightforward/safe way to make it
> > >    visible...
> > 
> 
> > Oh, sorry - I guess it is getting written out, but it's only
> > available via an O_DIRECT read from userspace; it's not
> > invalidating the cache.
> > 
> > # io/xfs_io -c "label derp" /mnt/test
> > label = "derp"
> > 
> > # dd if=/dev/sdb2 bs=512 count=1 | hexdump -C
> > ...
> > 00000060  00 00 0a 00 b4 e5 02 00  02 00 00 08 66 6f 6f 00  |............foo.|
> > ...
> > 
> > # dd if=/dev/sdb2 iflag=direct bs=512 count=1 | hexdump -C
> > ...
> > 00000060  00 00 0a 00 b4 e5 02 00  02 00 00 08 64 65 72 70  |............derp|
> > ...
> > 
> > # dd if=/dev/sdb2 bs=512 count=1 | hexdump -C
> > ...
> > 00000060  00 00 0a 00 b4 e5 02 00  02 00 00 08 66 6f 6f 00  |............foo.|
> > ...
> > 
> > Guess I need to think about this some more.
> > 
> 
> Isn't this to be expected? You're directly accessing the block device of
> a mounted filesystem. I would think this is expected behavior, so long
> as the set/get interfaces through the fs are consistent.

Trouble is, I bet blkid prints LABEL=foo here and not LABEL=derp as
the admin is probably expecting.

/me wonders if invalidate_inode_pages2_range here would help?

--D

> 
> Brian
> 
> > -Eric
> > 
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