mount: handle loopback mount by bergwolf · Pull Request #4178 · containerd/containerd

bergwolf · 2020-04-10T09:38:22Z

If a mount has specified loop option, we need to handle it on our
own instead of passing it to the kernel. In such case, create a
loopback device, attach the mount source to it, and mount the loopback
device rather than the mount source.

This is a spit from #3130 so that rawblock can be a non-builtin snapshotter.

/cc @dmcgowan @AkihiroSuda

theopenlab-ci · 2020-04-10T09:43:42Z

Build succeeded.

containerd-build-arm64 : FAILURE in 4m 38s (non-voting)

stevvooe · 2020-04-10T21:39:49Z

Would it be possible to reconcile this code with the other losetup code in containerd? https://github.com/containerd/containerd/search?q=losetup&unscoped_q=losetup provides some references. I didn't look into how similar they are.

theopenlab-ci · 2020-04-14T07:45:26Z

Build succeeded.

containerd-build-arm64 : NODE_FAILURE in 0s (non-voting)

theopenlab-ci · 2020-04-14T08:00:20Z

Build succeeded.

containerd-build-arm64 : NODE_FAILURE in 0s (non-voting)

bergwolf · 2020-04-14T08:04:24Z

@stevvooe I've implemented the same losetup APIs needed by devmapper, and replaced its private losetup command line wrapper with mount/losetup. PTAL.

bergwolf · 2020-04-14T08:04:56Z

btw, the failed ci looks unrelated:

##[error]github.com/containerd/containerd/api/events/container.proto:6:1: warning: Import gogoproto/gogo.proto is unused.
##[error]github.com/containerd/containerd/api/events/namespace.proto:5:1: warning: Import gogoproto/gogo.proto is unused.
panic: protobuf tag not enough fields in FileDescriptorSet.state:

estesp · 2020-04-14T19:11:15Z

We had a problem with unpinned dependencies breaking our protobuf checks in CI; if you can rebase on master HEAD, you should get a clean CI run now. Thanks!

mxpv · 2020-04-14T20:34:22Z

What's the benefit of having this losetup code vs the old package ?

The main purpose of the PR is to enable mount package to handle loopback mounts so that I can use it for the rawblock snapshotter. Maintainers also want to consolidate the two losetup implementation so I added the same APIs to replace the losetup command line wrappers. The main difference between them is calling the losetup binary or not.

Sorry, I meant why not just reuse existing package. Potentially this one may be a lot harder to maintain and support, since Go is not the best fit for low level programming.

Because it is not possible to create an autoclear loopback device with losetup command line. The workflow I want is:

open the loop device

set its backend and set the autoclear flag

mount the loopback device with proper file system

close the initial open file descriptor that we got in step 1

This is basically what you get with mount -o loop but it is not possible with the losetup command line. And the benefit is worthwhile -- we don't have to care about deleting the loopback device after unmounting the file system, nor if anything goes wrong in the middle before we mount the file system. The kernel takes care of cleaning up the loopback device once the last opener to it closes its open file descriptor.

And we're already calling mount syscall instead of the mount binary in the mount package. Doing the same for loopback device follows the same philosophy.

Because it is not possible to create an autoclear loopback device with losetup command line.

hm. As far as I understand losetup does that with --detach flag.

-d, --detach loopdev... Detach the file or device associated with the specified loop device(s). Note that since Linux v3.7 kernel uses "lazy device destruction". The detach operation does not return EBUSY error anymore if device is actively used by system, but it is marked by autoclear flag and destroyed later.

http://man7.org/linux/man-pages/man8/losetup.8.html

It is the detach action, not setting a loop device as autoclear. With autoclear, the loop device is detached automatically after it is unmounted -- that means we do not need to call losetup -d after unmounting a loop device.

If we use losetup command line, the workflow is like:

losetup -f <backing-file>

mount with a file system

unmount the file system

losetup -d <loop-device>

Whereas with autoclear, we do:

mount -o loop <backing-file> alike with mount package

umount the file system

Or do you mean that we should do the following?

losetup -f <backing-file>

mount with a file system

losetup -d <loop-device>

unmount the file system

That still doesn't look as neat as just doing

mount -o loop <backing-file> alike with mount package

umount the file system

theopenlab-ci · 2020-04-15T02:10:31Z

Build succeeded.

containerd-build-arm64 : NODE_FAILURE in 0s (non-voting)

bergwolf · 2020-04-15T02:11:27Z

@estesp Thanks for the hint. I've rebased on top of master.

If a mount has specified `loop` option, we need to handle it on our own instead of passing it to the kernel. In such case, create a loopback device, attach the mount source to it, and mount the loopback device rather than the mount source. Signed-off-by: Peng Tao <[email protected]>

No need to use the private losetup command line wrapper package. The generic package provides the same functionality. Signed-off-by: Peng Tao <[email protected]>

theopenlab-ci · 2020-04-16T09:00:30Z

Build succeeded.

containerd-build-arm64 : FAILURE in 4m 47s (non-voting)

zhsj

I think you don't need to define some magic values which are defined in golang.org/x/sys package.

zhsj · 2020-05-21T15:06:52Z

+}
+
+// struct loop_info64 in util-linux/include/loopdev.h
+type loopInfo struct {


I think you can use https://godoc.org/golang.org/x/sys/unix#LoopInfo64

zhsj · 2020-05-21T15:20:26Z

+	_ [112]byte
+}
+
+func ioctl(fd, req, args uintptr) (uintptr, uintptr, error) {


maybe using Ioctl* family from golang.org/x/sys/unix?

zhsj · 2020-05-21T15:22:07Z

+	ioctlSetFd       = 0x4C00
+	ioctlClrFd       = 0x4C01
+	ioctlSetStatus64 = 0x4C04
+	ioctlGetFree     = 0x4C82


unix.LOOP_SET_FD
unix.LOOP_CLR_FD
unix.LOOP_SET_STATUS64
unix.LOOP_CTL_GET_FREE

zhsj · 2020-05-21T15:22:58Z

+	//loFlagsUseAops   = 2
+	loFlagsAutoclear = 4
+	//loFlagsPartScan  = 8
+	loFlagsDirectIO = 16


unix.LO_FLAGS_READ_ONLY
unix.LO_FLAGS_AUTOCLEAR
unix.LO_FLAGS_PARTSCAN
unix.LO_FLAGS_DIRECT_IO

zhsj · 2020-05-21T15:27:41Z

+	//loFlagsPartScan  = 8
+	loFlagsDirectIO = 16
+
+	ebusyString = "device or resource busy"


unix.EBUSY.Error() is this. But I think you don't need to compare the error string.

mxpv · 2020-06-25T21:53:58Z

+1 for using existing routines from unix package. Could you possibly address that @bergwolf ?

AkihiroSuda · 2020-08-04T03:47:34Z

ping @bergwolf

Signed-off-by: Maksym Pavlenko <[email protected]>

mxpv · 2021-01-08T22:24:26Z

Carried in #4902

bergwolf mentioned this pull request Apr 10, 2020

Introduce the rawblock snapshotter #3130

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AkihiroSuda reviewed Apr 10, 2020

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Comment thread mount/losetup_linux.go Outdated

bergwolf force-pushed the losetup branch from 3a3d274 to bacb3c7 Compare April 14, 2020 07:44

bergwolf force-pushed the losetup branch from bacb3c7 to bef0b0a Compare April 14, 2020 07:59

mxpv reviewed Apr 14, 2020

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bergwolf force-pushed the losetup branch from bef0b0a to 24c3cc7 Compare April 15, 2020 02:10

bergwolf added 2 commits April 16, 2020 01:53

snapshot/devmapper: use losetup in mount package

44af9b9

No need to use the private losetup command line wrapper package. The generic package provides the same functionality. Signed-off-by: Peng Tao <[email protected]>

bergwolf force-pushed the losetup branch from 24c3cc7 to 44af9b9 Compare April 16, 2020 08:54

zhsj reviewed May 21, 2020

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dmcgowan assigned mxpv Jun 25, 2020

mxpv added a commit to mxpv/containerd that referenced this pull request Jan 4, 2021

Address loop dev PR comments containerd#4178

c5fa029

Signed-off-by: Maksym Pavlenko <[email protected]>

mxpv mentioned this pull request Jan 4, 2021

[Carry] mount: handle loopback mount #4902

Merged

mxpv closed this Jan 8, 2021

bergwolf deleted the losetup branch November 7, 2022 03:05

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