[PATCH v2 0/4] xfs: fix v5 AGFL wrapping

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

Hi all,

Here's a bunch of patches fixing the AGFL padding problem once and for
all.  When the v5 disk format was rolled out, the AGFL header definition
had a different padding size on 32-bit vs 64-bit systems, with the
result that XFS_AGFL_SIZE reports different maximum lengths depending on
the compiler.  In Linux 4.5 we fixed the structure definition, but this
has lead to sporadic corruption reports on filesystems that were
unmounted with a pre-4.5 kernel and a wrapped AGFL and then remounted on
a 4.5+ kernel.

To deal with these corruption problems, we introduce a new ROCOMPAT
feature bit to indicate that the AGFL has been scanned and guaranteed
not to wrap.  We then amend the mounting code to find broken wrapping,
fix the wrapping, and if we had to fix anything, set the new ROCOMPAT
flag.  The ROCOMPAT flag prevents re-mounting on unpatched kernels, so
this series will likely have to be backported.

--D