btrfs-safe implementation of -oloop
Status from this message: Applied, thanks.
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Patches in v1 Applied
| # | Subject and files | Size | Replies | Applied |
|---|---|---|---|---|
| 1/3 | libmount: Re-organize is_mounted_same_loopfile()libmount/ | +14 | 0 replies | |
| 2/3 | libmount: reuse existing loop devicelibmount/ | +24 | 0 replies | |
| 3/3 | mount: Handle EROFS before calling mount() syscallsys-utils/ | +9 | 0 replies |
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Cover letter [PATCH 0/3] btrfs-safe implementation of -oloop
As it was discussed in LKML[1], kernel has no way to detect backing file data changes if multiple loop devices have the same backing file.
Exactly this happened if -oloop is used multiple times with the same source.
It could not happen in past, but with introduction of btrfs sub-volumes it has a perfectly legal use cases.
This patch set introduces a new behavior of -oloop:
First check, whether the same backing file with the same offset is already used. If yes, reuse already initialized loop device.
If not, initialize another loop device.
There are some controversial things with these patches. Some of them cannot be fixed.
- Kernel never returns EBUSY for LOOP_CLR_FD. It prevents errors when mount will steal other's loop device. But I am not sure, whether it will work correctly in all possible situations. Anyway, I don't see a better solution.
- If the same backing file is already used for a read-only loop device, there is no safe way to continue.
- There is no way to turn R/O loop to R/W in kernel.
- If another loop is initialized, changes will not propagate to R/O volume.
- One would need to umount all R/O devices, initialize loop R/W, and then everything mount again.
I can imagine partial solution: Introduce looprw option. Such option would cause to initialize loop device R/W even for R/O mount.
- If the same backing file is already used for a loop device with correct offset, but incorrect sizelimit, there is no solution. The current implementation does not check for it.
- There exists a change for a race condition between device lookup and mount syscall.
- The implementation does not check for crypto. I think it is not a big problem, as it makes no sense to initialize the same backing file as encrypted and non-encrypted at once.
References:
[1] https://lkml.org/lkml/2016/2/26/897
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Best Regards / S pozdravem, Stanislav Brabec software developer --------------------------------------------------------------------- SUSE LINUX, s. r. o. e-mail: [email protected] Lihovarská 1060/12 tel: +49 911 7405384547 190 00 Praha 9 fax: +420 284 084 001 Czech Republic http://www.suse.cz/ PGP: 830B 40D5 9E05 35D8 5E27 6FA3 717C 209F A04F CD76