On Thu, Dec 24, 2015 at 10:22:31AM +1100, Stewart Smith wrote:

Eric Sandeen [off-list ref] writes:

quoted

quoted

3) A lot of user even don't now mount ro can still modify device
   Yes, I didn't know this point until I checked the log replay code of
   btrfs.
   Adding such mount option alias may raise some attention of users.

Given that nothing in the documentation implies that the block device itself
must remain unchanged on a read-only mount, I don't see any problem which
needs fixing.  MS_RDONLY rejects user IO; that's all.

If you want to be sure your block device rejects all IO for forensics or
what have you, I'd suggest # blockdev --setro /dev/whatever prior to mount,
and take it out of the filesystem's control.  Or better yet, making an
image and not touching the original.

What we do for the petitboot bootloader in POWER and OpenPower firmware
(a linux+initramfs that does kexec to boot) is that we use device mapper
to make a snapshot in memory where we run recovery (for some
filesystems, notably XFS is different due to journal not being endian
safe). We also have to have an option *not* to do that, just in case
there's a bug in journal replay... and we're lucky in the fact that we
probably do have enough memory to complete replay, this solution could
be completely impossible on lower memory machines.

Which means the boot loader is going to break horribly when we
change the on-disk format and feature flags the boot loader doesn't
understand get set in the root filesystem. Then the bootloader will
refuse to mount the filesystem and the system won't boot anymore...

IOWs, developers and users can't make a root XFS filesystem with a
new/experimental feature on POWER/OpenPower machines because the
bootloader will refuse to mount it regardless of the clean/dirty
state of the journal....

As such, I believe we're the only bit of firmware/bootloader ever that
has correctly parsed a journalling filesystem.

Nope.  The Irix bootloader (sash) could do this 20 years ago - there
are even feature mask bits reserved specifically for SASH in the XFS
superblock. However, seeing as the bootloader was always upgraded
during the install of each new Irix release, the bootloader was
always up-to-date with the on-disk features the kernel supported and
so there was never a problem with mismatched feature support.

However, Linux users can upgrade or change the kernel at any time
independently of the bootloader, so it's pretty much guaranteed that
mismatched bootloader/kernel filesystem capabilities will cause
users problems at some point in the not-too-distant future.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com