On Wed, Dec 16, 2015 at 09:15:59PM -0600, Eric Sandeen wrote:

<xfs list address fixed>

On 12/16/15 7:41 PM, Qu Wenruo wrote:

quoted

Hi,

In a recent btrfs patch, it is going to add a mount option to disable
log replay for btrfs, just like "norecovery" for ext4/xfs.

But in the discussion on the mount option name and use case, it seems
better to have an unified and fs independent mount option alias for
real RO mount

Reasons:
1) Some file system may have already used [no]"recovery" mount option
   In fact, btrfs has already used "recovery" mount option.
   Using "norecovery" mount option will be quite confusing for btrfs.

Too bad btrfs picked those semantics when "norecovery" has existed on
other filesystems for quite some time with a different meaning... :(

quoted

2) More straight forward mount option
   Currently, to get real RO mount, for ext4/xfs, user must use -o
   ro,norecovery.
   Just ro won't ensure real RO, and norecovery can't be used alone.
   If we have a simple alias, it would be much better for user to use.
   (it maybe done just in user space mount)

mount(8) simply says:

       ro     Mount the filesystem read-only.

and mount(2) is no more illustrative:

       MS_RDONLY
              Mount file system read-only.

kernel code is no help, either:

#define MS_RDONLY        1      /* Mount read-only */

They say nothing about what, exactly, "read-only" means.  But since at least
the early ext3 days, it means that you cannot write through the filesystem, not
that the filesystem will leave the block device unmodified when it mounts.

I have always interpreted it as simply "no user changes to the filesystem,"
and that is clearly what the vfs does with the flag...

That ("-o ro means no user changes") has always been my understanding too.  You
/want/ the FS to replay the journal on an RO mount so that regular FS operation
picks up the committed transactions.

--D

quoted

   Not to mention some fs (yeah, btrfs again) doesn't have "norecovery"
   but "nologreplay".

well, again, btrfs picked unfortunate semantics, given the precedent set
by other filesystems.

f2fs, ext4, gfs2, nilfs2, and xfs all support "norecovery" - xfs since
forever, ext4 & f2fs since 2009, etc.

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.

-Eric

quoted

Any ideas about this?

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