Temporarily remounting rootfs as rw leads to kernel panic on reboot

[David Olson]

> > > UBI error: ubi_io_read: error -74 (ECC error) while reading 126976
> > > bytes from PEB 3:4096, read 126976 bytes UBI error: ubi_io_read:
> > > error
> > > -74 (ECC error) while reading 126976 bytes from PEB 4:4096, read
> > > 126976 bytes UBI error: ubi_io_read: error -74 (ECC error) while
> > > reading 11 bytes from PEB 10:10240, read 11 bytes UBIFS error (pid 1):
> > > ubifs_leb_read: reading 11 bytes from LEB 8:6144 failed, error -74

The root cause has been determined and was the result of two changes, one
deliberate and one accidental.

Since we write the UBIFS rootfs to NAND via commands from a version of
U-Boot that doesn't support "nand write.trimffs", we use the "-F" flag in the
invocation of mksf.ubifs.  It turns out that the "-F" flag (at least in the version
of UBIFS we are using) does NOT cause the requested "free space fixup"
operation on the first mount if the mount is not r/w.  For our system,
rootfs is mounted twice, once using parameters passed in from U-Boot on the
kernel command line and later when the kernel processes /etc/fstab.  While
the latter requested a ro mount, the former, until late in the project, requested
a rw mount.  Changing the kernel command line to request a ro mount was a
deliberate change late in the project.  The other change was the inadvertent
loss of the "-F" flag when we ported from the TI provide "makefile based" build
to a Yocto "bitbake based" build.  Since the "-F" flag is ineffective when the first
mount is ro, we didn't notice the change.  Undoing both those changes has
resolved our issue.

While I now understand why a write to our rootfs in the absence of the "free
space fixup" operation would corrupt it, I still don't understand why just
changing the mount parameters would cause a write.  But I guess it no longer
matters.


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