Eric Sandeen wrote on 2015/12/16 21:15 -0600:
<xfs list address fixed>
On 12/16/15 7:41 PM, Qu Wenruo wrote:
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... :(
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...
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.
I understand it's btrfs' fault.
Considering how many filesystems are already using "norecovery", it is
almost a standard.
Not sure if it's possible to change the "recovery" mount option to other
name for btrfs, but it seems using "norecovery" would be the best solution.
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.
And thanks for the info provided by Karel, it's clear that at least
mount(8) itself already has explain on what ro will do and what it won't do.
Thanks,
Qu
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
Any ideas about this?
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