Re: [PATCH v3 4/5] xfs_db: sanitize geometry on load

[Eric Sandeen <sandeen@redhat.com>]

On 1/13/17 7:35 AM, Brian Foster wrote:
>> +		fprintf(stderr,
>> +_("%s: device %s AG count is insane, but could be %u.  Limiting reads to AG 0.\n"),
>> +			progname, fsdevice, dblocks / sbp->sb_agblocks);
>> +	} else {
>> +		fprintf(stderr,
>> +_("%s: device %s AG count is insane.  Limiting reads to AG 0.\n"),
>> +			progname, fsdevice);
>> +	}
> For reasons like the above, I think xfs_db shouldn't be in the business
> of repair like validation (xfs_check notwithstanding). That said,
> dropping into a fixed single AG mode seems less risky than trying to
> surmise a valid geometry. I'd get rid of the "this might be your
> agcount" messaging entirely though and just replace it with something
> that explicitly states the filesystem is corrupted, the runtime geometry
> is invalid and that the user should probably run xfs_repair before doing
> anything.

So keep in mind that xfs_db is for people with super xfs powers. (*)

I wouldn't suggest repair, I'd start with 1 ag to avoid the OOM, state 
that clearly, and punt the problem to the admin with no other specific
suggestions.

> I still like the idea of the single AG mode thing as a command line flag
> rather than default behavior because it requires user acknowledgement,
> but this is a debug tool after all, so I'll defer to Eric on that. I do
> think that if we create this kind of invalid runtime mode, this should
> be split into two patches. First, a bugfix patch for the core OOM
> problem (i.e., detect a wacky superblock and exit). Second, replace the
> exit with the single AG runtime mode thing.

Well, the problem with a flag, I think, is that you might have already
unwittingly OOMed your box to find out that you need it.
Rebooting to try again with a flag sucks.

(*) unless you are invoking it via xfs_admin.sh, dammit.  We sure wouldn't
want xfs_admin to exit happily, having updated only one AG.  Dammit!

Perhaps it should set exitcode, and then xfs_admin could do something
like:

	xfs_db -c quit $DEV

first, and check that db is able to initialize sanely before using it again
to perform normal admin functions.

-Eric