[J/L][PATCH 0/2] loop: fix regression from max_loop default value change https://lists.ubuntu.com/archives/kernel-team/2023-July/141350.html
-- You received this bug notification because you are a member of Kernel Packages, which is subscribed to linux-hwe-5.15 in Ubuntu. https://bugs.launchpad.net/bugs/2015400 Title: losetup with mknod fails on jammy with kernel 5.15.0-69-generic Status in linux package in Ubuntu: Invalid Status in linux-hwe-5.15 package in Ubuntu: Invalid Status in linux source package in Focal: Invalid Status in linux-hwe-5.15 source package in Focal: In Progress Status in linux source package in Jammy: In Progress Status in linux-hwe-5.15 source package in Jammy: Invalid Status in linux source package in Lunar: In Progress Status in linux-hwe-5.15 source package in Lunar: Invalid Status in linux source package in Mantic: Invalid Status in linux-hwe-5.15 source package in Mantic: Invalid Bug description: [ Impact ] * Regression in the loop block driver in Jammy kernel between 5.15.0-67 to 5.15.0-68 (in v5.15.86 stable), due to a change in the default behavior (value) of kernel parameter `max_loop` (from 0 to 8) in commit: `loop: Fix the max_loop commandline argument treatment when it is set to 0` (comment #6). * Users of loop devices (major 7) with minor >= 8 now fail to `open()` a loop device created with `mknod()`. * This is a corner case, as most people use `losetup` with usual /dev/loopNUMBER (or `--find`) which are not affected as it uses a different code path. (`losetup` for `/dev/loopNOT-A-NUMBER` is affected.) * Workaround: kernel parameter `max_loop=0`. [ Test Steps ] * Run the test cases (losetup and test-loop.c in comment #6): - max_loop not set (default) - max_loop=0 - max_loop=8 * Verify the default behavior (max_loop not set) is restored. * Verify the modified behavior (max_loop is set) is unchanged. [ Regression Potential ] * Regressions would be limited to the loop block driver, more specifically its default behavior (but it's that now) or specific usage of max_loop parameter (tested; looks OK). [ Other Info ] * The fix on Jammy is just a Revert, since it had not been released with the offending patch. * The fix on Lunar is the recently accepted 2 patches [1, 2] as it was released with the offending patch, so let's keep that patch's improvement/behavior for the max_loop=0 case, and "fix"/restore the historical no-limit default behavior. * The fix on Mantic is the recently accepted 2 patches too, now in v6.5-rc3, which should be automatically incorporated as Mantic apparently will release with the 6.5 kernel [3] * Patch 1 [1] is not quite a fix, but adds CONFIG guards that Patch 2 [2] depends on. (Alternatively, a Patch 2 backport with that could be done, but Patch 1 seems trivial enough.) [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=23881aec85f3219e8462e87c708815ee2cd82358 [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bb5faa99f0ce40756ab7bbbce4f16c01ca5ebd5a [3] https://discourse.ubuntu.com/t/introducing-kernel-6-5-for-the-23-10-mantic-minotaur-release Original Description: --- losetup fails with devices created manually by mknod on kernel 5.15.0-69-generic # fallocate -l 1G test # mknod -m 660 /dev/loop8 b 7 8 # chown root:disk /dev/loop8 # losetup /dev/loop8 ./test losetup: ./test: failed to set up loop device: Device or resource busy Possibly as a result of this patch: https://lore.kernel.org/lkml/20221208200605.756287-1-isaacmanjar...@google.com/T/ which was introduced in 5.15.0-68: http://launchpadlibrarian.net/653145495/linux_5.15.0-67.74_5.15.0-68.75.diff.gz On a machine prior to this change (no issue with losetup): # cat /sys/module/loop/parameters/max_loop 0 # uname -r 5.15.0-58-generic On a machine after the change (has losetup issue as described above): # cat /sys/module/loop/parameters/max_loop 8 # uname -r 5.15.0-69-generic So it looks like the default changed and the max amount of loop devices that can be created with mknod (not loop-control) is 8. If we set max_loop=0 on the kernel command line, it works as before. Cannot unload and reload module on a running system to change the parameter because it is built into the kernel. Another workaround is to use `losetup --find` but that means you cannot create with named devices created with mknod. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2015400/+subscriptions -- Mailing list: https://launchpad.net/~kernel-packages Post to : kernel-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~kernel-packages More help : https://help.launchpad.net/ListHelp
