Re: [RFC-PATCH] xfs: do not update sunit/swidth in the superblock to match those provided during mount

[Brian Foster <bfoster@redhat.com>]

On Sun, Nov 24, 2019 at 11:38:53AM -0600, Eric Sandeen wrote:
> On 11/24/19 10:40 AM, Darrick J. Wong wrote:
> > On Sun, Nov 24, 2019 at 11:13:09AM +0200, Alex Lyakas wrote:
> 
> ...
> 
> >>>> With the suggested patch, xfs repair is working properly also when mount-provided sunit/swidth are different.
> >>>>
> >>>> However, I am not sure whether this is the proper approach.
> >>>> Otherwise, should we not allow specifying different sunit/swidth
> >>>> during mount?
> > 
> > I propose a (somewhat) different solution to this problem:
> > 
> > Port to libxfs the code that determines where mkfs/repair expect the
> > root inode.  Whenever we want to update the geometry information in the
> > superblock from mount options, we can test the new ones to see if that
> > would cause sb_rootino to change.  If there's no change, we update
> > everything like we do now.  If it would change, either we run with those
> > parameters incore only (which I think is possible for su/sw?) or refuse
> > them (because corruption is bad).
> > 
> > This way we don't lose the su/sw updating behavior we have now, and we
> > also gain the ability to shut down an entire class of accidental sb
> > geometry corruptions.
> 

Indeed, I was thinking about something similar with regard to
validation. ISTM that we either need some form of runtime validation...

> I also wonder if we should be putting so much weight on the root inode
> location in repair, or if we could get away with other consistency checks
> to be sure it's legit, since we've always been able to move the
> "expected" Location.
> 

... or to fix xfs_repair. ;) Fixing the latter seems ideal to me, but
I'm not sure how involved that is compared to a runtime fix. Clearly the
existing repair check is not a sufficient corruption check on its own.
Perhaps we could validate the inode pointed to by the superblock in
general and if that survives, verify it looks like a root directory..?
The unexpected location thing could still be a (i.e. bad alignment)
warning, but that's probably a separate topic.

I'm not opposed to changing runtime behavior even with a repair fix,
fwiw. I wonder if conditionally updating the superblock is the right
behavior as it might be either too subtle for users or too disruptive if
some appliance out there happens to use a mount cycle to update su/sw.
Failing the mount seems preferable, but raises similar questions wrt to
changing behavior. Yes, it is corruption otherwise, but unless I'm
missing something it seems like a pretty rare corner case (e.g. how many
people change alignment like this? of those that do, how many ever run
xfs_repair?). To me, the ideal behavior is for mount options to always
dictate runtime behavior and for a separate admin tool or script to make
persistent changes (with associated validation) to the superblock.

Brian