RE: [RFC][PATCH] fs: configfs: programmatically create config groups

[Andrzej Pietrasiewicz]

On Monday, December 10, 2012 3:34 PM Felipe Balbi wrote:
> Subject: Re: [RFC][PATCH] fs: configfs: programmatically create config
> groups
> 
> Hi,
> 
> On Mon, Dec 10, 2012 at 03:17:34PM +0100, Andrzej Pietrasiewicz wrote:
> > @Joel in particular: please see my comment in the bottom.
> >
> > On Monday, December 10, 2012 12:57 PM I wrote:
> > > Subject: RE: [RFC][PATCH] fs: configfs: programmatically create
> > > config groups
> > >
> > > Hello Joel,
> > >
> > > So you are alive, I'm glad to hear from you ;) Thank you for your
> > response.
> > >
> > > On Saturday, December 08, 2012 12:18 AM Joel Becker wrote:
> > > > Subject: Re: [RFC][PATCH] fs: configfs: programmatically create
> > > > config groups
> > > >
> > > > Hey Guys,
> > > > 	Sorry I missed this for a while.  I'll make a couple of
inline
> > > > comments, and then I'll summarize my (incomplete) thoughts at the
> > bottom.
> > > >
> > > > On Wed, Nov 28, 2012 at 02:50:13PM +0100, Sebastian Andrzej
> > > > Siewior
> > > wrote:
> > > > > On 11/28/2012 02:05 PM, Michal Nazarewicz wrote:
> > > > > >>On 11/27/2012 05:23 PM, Michal Nazarewicz wrote:
> > > > > >>>How should a generic tool know what kind of actions are
> > > > > >>>needed for given function to be removed?  If you ask me,
> > > > > >>>there should be a way to unbind gadget and unload all modules
> > > > > >>>without any specific knowledge of the functions.  If there is
> > > > > >>>no such mechanism, then it's a bad user interface.
> > > >
> > > > 	Please remember that configfs is not a "user" interface,
it's
> a
> > > > userspace<->kernelspace interface.  Like sysfs, it's not required
> > > > to be convenient for someone at a bash prompt.  My goal is that it
> > > > is *usable* from a bash prompt.  So it must be that you can
> > > > create/destroy/configure objects via mkdir/rmkdir/cat/echo, but
> > > > you might have a lot of those mkdir/echo combos to configure
> something.
> > > > When it comes to the "user" interface, a wrapper script or library
> > > should
> > > > be converting a user intention into all that boilerplate.
> > > >
> > >
> > > If you say so there is little we can do, is there? :O
> > >
> > > <snip>
> > >
> > > >
> > > > 	Yeah, user tools are expected (and should be).
> > > >
> > >
> > > ditto
> > >
> > > > > Anyway, for this to work we have to go through Joel.
> > > > >
> > > > > >	rmdir udcs/*		# unload all UDCs
> > > > >
> > > > > No, for this you still have to rmmod :)
> > > > >
> > > > >
> > > > > >>>I think the question is of information flow direction.  If
> > > > > >>>user gives some information to the kernel, she should be the
> > > > > >>>one creating any necessary directories.  But if the
> > > > > >>>information comes from kernel to the user, the kernel should
> create the structure.
> > > >
> > > > 	This last paragraph actually describes the distinction
between
> > > > configfs and sysfs.  More specifically, if userspace wants to
> > > > create an object in the kernel, it should use configfs.  If the
> > > > kernel has created an object on its own, it exposes it via sysfs.
> > > > It is a deliberate non- goal for configfs to replicate sysfs
> behavior.
> > >
> > > So if the lifetime of some object is controlled by the user, it
> > > belongs to configfs. If, on the other hand, the lifetime is
> > > controlled by the kernel, it belongs to sysfs. I think that the
> > > trouble with e.g. luns is that they might want to behave like both
> > > (at least in the "more automated"
> > > approach where they are programmatically created): they are created
> > > by the kernel (=> sysfs) but their lifetime is controlled by the
> > > user (=>configfs).
> > >
> > > >
> > > > [General Thoughts]
> > > >
> > > > 	First let me restate your problem to see if I understand it.
> > > > 	You want to expose e.g. five LUNs.  They should eventually
> appear
> > > > in configfs as five items to configure (.../{lun0,lun1,...lun4}/.
> > > > The current configfs way to do this is to have your setup script do:
> > > >
> > > > 	cd /cfg/.../mass_storage
> > > > 	mkdir lun0
> > > > 	echo blah >lun0/attr1
> > > > 	echo blahh >lun0/attr2
> > > > 	mkdir lun1
> > > > 	echo blag >lun1/attr1
> > > > 	echo blagg >lun1/attr1
> > > > 	...
> > > >
> > > > I think the primary concern expressed by Andrzej is that a random
> > > > user could come along and say "mkdir lun8", even though the gadget
> > > > cannot support it.  A secondary concern from Michal is that
> > > > userspace has to
> > > run
> > > > all of those mkdirs.  The thread has described varying solutions.
> > > > 	If these original directories were default_groups, you could
> > > > disallow any child creation and not require the setup script to
> > > > create directories.  Here I will point out that you *can* create
> > > > default_groups programmatically.  The default_groups pointer on
> > > > each configfs_group can be different.  ->make_group() can attach
> whatever groups it wants.
> > > > If this would work, it would fit both of your concerns, but you do
> > > > not have a priori knowledge of the LUN count.
> > > > 	Your original email proposed that the max lun be set like
so:
> > > >
> > > > 	cd /cfg/.../mass_storage
> > > > 	echo 5 >luns_allowed
> > > >
> > > > There are then a couple of proposed ways to enforce the limit.
> > > > Your
> > > > ->create_group() idea wants luns_allowed to be able to create
> > > > ->subdirs in
> > > > the ->store() function.  No ->mkdir() is provided, so a lunN
> > > > cannot be created by hand.
> > > > 	The ->create_group() idea has three challenges.  First, it
> > > > creates "default" groups *after* the object is created.  This
> > > > makes no sense if they are attributes of the toplevel object.
> > > > Second, it volates the configfs mantra that user-generated objects
> are explicitly created.
> > > > The kicker, however, is the technical problem.  configfs
> > > > explicitly forbids tree changes inside attribute operations for
> deadlock reasons.
> > > > Trying to create new tree parts in luns_allowed->store() is
> > > > exactly
> > that.
> > > > 	Another suggestion (I think I read this in the thread) would
> be
> > > > to have ->mkdir() function parse the name and check that it is
> > > > lunN, where
> > > N
> > > > is less than the limit.  I think that this was shot down because
> > > > of parsing the LUN name.  I don't think that's too terrible, but
> > > > there's certainly room for argument.
> > > >
> > > > [Possible Solutions]
> > > >
> > > > 	Before coming back to the proposed patch, let's look at what
I
> > > > might do if I were fitting this into configfs.
> > > > 	I would actually do what I described at first:
> > > >
> > > > 	cd /cfg/.../mass_storage
> > > > 	mkdir lun0
> > > > 	echo blah >lun0/attr1
> > > > 	...
> > > >
> > > > This is idiomatic configfs usage.  Sebastian noted this earlier in
> > > > the thread.  Michal thought the repeated mkdirs were a problem ("I
> > > > think that's suboptimal, and we can do better").  This is Michal's
> > > > objection
> > > to
> > > > making the user do work.  This is where I point out we are not
> > > > designing configfs interfaces for ease of end-user use.  We're
> > > > defining them for transparent and discoverable interfaces for
> > > > tools to create things in
> > > the
> > > > kernel.  So the repeated mkdir()s are actually a good thing from
> > > > my perspective; userspace is telling the kernel to create LUN
> objects.
> > > > 	But this doesn't alleviate Andrzej's primary concern, nor
does
> it
> > > > answer Michal's concerns about corner cases.  Let's see if we can
> > > > solve those.
> > > > 	First, what about preventing extraneous LUNs?  There are two
> > > > approaches: the prevent approach, and the ignore approach.  In the
> > > Target
> > > > module, they take the ignore approach.  If you create a LUN that
> > > > is outside the scope, they just ignore it.  So a user can make 10
> > > > LUNs, but if only five are allowed, the other five are ignored.
> > > > In the prevent approach, we try to fail the mkdir() when we go over
> the limit.
> > >
> > > I think I like the prevent approach better, because I don't know
> > > what to do with the unused luns in the ignore approach. I also think
> > > that it would be nice if configfs described the actual state of the
> > > system instead of presenting all the possible user-created garbage
> > > (i.e. something which is ignored).
> > >
> > > > 	So you've set a limit with "echo 5 >luns_allowed".  How do
you
> > > > restrict?  If you don't care about contiguous LUN numbering, you
> > > > could literally do this in your make_group():
> > > >
> > > > 	if (count_luns(parent) >= parent->luns_allowed)
> > > > 		return -ENOSPC;
> > > >
> > > > Note that we don't check the LUN number; SCSI does not require
> > > contiguous
> > > > numbering.  It does require LUN 0, which should probably be a
> > > > default group.  So your lun count starts at 1.
> > > > 	What if you decide you really want contiguous LUN numbers,
> which
> > > > is considered nice to have?  Then we get to Michal's concern about
> > > > parsing the name.  If you "mkdir lun1", the code has to confirm
> > > > that it
> > > > a) starts with "lun", b) finishes with a parsable number, c) that
> > > > number is < luns_allowed.
> > > > 	But you need to do the parsing anyway!  If the LUN is
created
> via
> > > > mkdir(), somehow the gadget system needs to know what LUN number
> > > > to advertise.  I would actually decouple it from the name.  The
> > > > name should be free-form; if it happens to be comprehensible,
> > > > that's good.  Imagine
> > > > this:
> > > > 	cd /cfg/.../mass_storage
> > > > 	echo 4 >max_lun_id
> > > > 	mkdir lun100
> > > > 	echo 1 >lun100/lun_id
> > > >
> > > > The LUN actually has the ID of 1.  The fact that the directory
> > > > name is wrong is irrelevant to the gadget processing; non-hackers
> > > > use the
> > > tooling,
> > > > and the tooling should  make sane names.  Then your
> > > > ->make_group() function can choose either approach to avoiding LUN
> > > > ->IDs
> > > > that are too large.  It can prevent:
> > > >
> > > > 	lun_id->store() {
> > > > 		if (lun_id_exists(parent, value))
> > > > 			return ERR_PTR(-EINVAL);
> > > > 		if (value > parent->max_lun_id)
> > > > 			return ERR_PTR(-ENOSPC) /* or EINVAL */
> > > > 		setup_lun();
> > > > 		make_live();
> > > > 	}
> > > >
> > > > Or it can ignore:
> > > >
> > > > 	lun_id->store() {
> > > > 		if (lun_id_exists(parent, value))
> > > > 			return ERR_PTR(-EINVAL);
> > > > 		if (value <= parent->max_lun_id) {
> > > > 			setup_lun();
> > > > 			make_live();
> > > > 		}
> > > > 		/* Do nothing for the ignored sucker */
> > > > 	}
> > > >
> > > > 	I think that I, personally, would go with this <name>/lun_id
> scheme.
> > > > Given a requirement for contiguous LUNs and the max_lun_id
> > > > restriction, I'd go with the "prevent" option myself.
> > >
> > > Taking into account what you say about the expected userspace
> > > tooling using a free-form name plus the lun_id attribute could be
> > > attractive, because the lun_id attribute's value can be analyzed at
> > > ->store() and, if it doesn't fit, an appropriate error can be easily
> > > returned using just what we already have in configfs. Prevent seems
> better to me, too.
> > >
> > > > 	But is max LUNs a requirement?  Why not just have the LUN
> count
> > > > be the number of LUNs created?  In that case, you can allow or
> > > > disallow discontiguous LUN IDs without worrying about a max ID.
> > > >
> > >
> > > I am not sure, but I think that Michal would say that he wouldn't
> > > like things to change once the gadget is up and running, so the
> > > number of luns should be fixed up front. On the other hand, after
> > > the gadget is set up, we can fail in ->make_group() to prevent the
> > > user from creating additional luns with which we don't know what to
> > > do. So what you suggest seems fine, but I would ask Michal to be on a
> safe side.
> > >
> > > > [Really Hate Mkdir]
> > > >
> > > > 	If you considered it appropriate for the LUN objects to be
> > > > created by the kernel, the standard way would to have "echo 5
> >luns_allowed"
> > > > create LUN objects in sysfs.  Yes, sysfs.
> > > > There's no reason you can't have things in configfs *and* sysfs.
> > > > My understanding of your code says to me that the mkdir approach
> > > > is the way to go, but if you hate it, this works.
> > > >
> > >
> > > I guess you are right, but see my point about luns' lifetime being
> > > controlled by the user and their creation being performed by the
> kernel.
> > >
> > > > [What About the Patch?]
> > > >
> > > > 	In general, attribute setting that turns into created
configfs
> > > > objects is against the theory of configfs.
> > >
> > > This looks like an authoritative answer.
> > >
> > > > In practice, it's a nightmare
> > > > locking change.  Coming into this discussion, I though you were
> > > > doing dymanic things at ->make_group() time, but that is already
> supported.
> > > > 	But I want to hear your thoughts.  I've dumped a bunch of
> > > > alternate approaches here.  Do any seem to fit?  What other
> challenges do you see?
> > > >
> > > > Joel
> > > >
> > >
> > > Once upon a time, Felipe expressed interest in having the
> > > information about endpoints and interfaces (mostly read-only stuff,
> > > but not all of it) accessible in the gadget subsystem in configfs.
> > > This kind of information is fully available only after the gadget
> > > has been bound, that is, after the user created all the groups/items
> > > and filled all the attributes and told the gadget to start (with
> > > e.g. filling some attribute or creating a symlink). So this looks
> > > like a programmatic creation of configfs directories. On the other
> > > hand, Felipe wanted it for debugging purposes, so the user could
> > > just as well create the required directories by hand when they need
> > > to, as Sebastian once suggested. Again, this requires
> > > ->make_group() to fail if all the information is not available yet.
> > > But isn't it kind of similar to the luns' case? It is a bunch of
> > > objects whose lifetime is controlled by the user (bind the gadget,
> > > unbind the gadget), but which are created by the kernel.
> > > Another problem with this is that the user must at some point
> > > _guess_ what name to use in mkdir. Example:
> > >
> > > For each interface there should be a folder. So the user must know
> > > how many interfaces' folders to create and how to name them, like:
> > >
> > > $ mkdir ...../some_usb_function/interface00
> > > $ mkdir ...../some_usb_function/interface01
> > > ...
> > > ...
> > > ...
> > >
> > > In each interface folder, there should be some endpoint folders and
> > > the user must know how many endpoints there are and how to name
> > > them:
> > >
> > > $ mkdir ...../some_usb_function/interface00/endpoint02
> > > $ mkdir ...../some_usb_function/interface00/endpoint81
> > >
> > > So probably there should be some attribute in some_usb_function,
> > > which contains a list of available interface names, and some
> > > attribute in interface#, which contains a list of available endpoint
> names.
> > > Workable, but not so convenient. And there could be a problem, too,
> > > if the gadget is unbound: there is no way to remove the user-created
> > > directories other than by the user themselves, so there will be
> > > times (i.e. between gadget unbind and manual rmdir) when these
> > > interface/endpoint directories will not correspond to anything
> > > present in the system.
> > >
> > > So maybe this kind of information should live in sysfs? But I don't
> > > know now if it would be any easier. Joel? Felipe?
> > >
> >
> > I forgot to mention, representing udcs (USB Device Controllers) in
> > configfs is similar to interfaces/endpoints: the user needs to guess
> > what name to use in mkdir, e.g. in:
> >
> > $ mkdir ....../udcs/s3c-hsotg
> 
> why would you even require a UDC directory to be created ? You
> *register* the UDC with the udc-class and that should be enough to expose
> all UDCs under /sys/kernel/config/..../udcs/*

Because in configfs the only way for something to appear under e.g.
/...../config/udcs is to create a directory with mkdir. This is
what Joel says.

Except perhaps for default groups which *must* be known at the
moment of creation of their parent directory. So, when the
/....../config/udcs wants to appear, its default groups,
if any, must be known at this point. I'm not sure if it can be
guaranteed that when the /...../config/udcs is created all
the UDCs are known (*registered*, if you will). Plus, UDCs
can be compiled as modules. If a UDC driver module is inserted, e.g.:

$ modprobe s3c-hsotg

but the /...../config/udcs directory already exists, then
there is no way for the new udc to appear there if there is no
programmatic directory creation. And it seems there will not
be. Ergo, mkdir must be used and the user *needs* to know
what name to use.

> 
> The fact is that in most cases you will have multiple instances of the
> *same IP* not different UDCs with different names as you seem to think.
> While that's possible, it'll be a rare situation.
> 

It doesn't matter if their directories need to be manually created with
mkdir.

> So what we will have in 95% of the cases will be:
> 
> /...../udcs/dwc3.0
> /...../udcs/dwc3.1
> ...
> /...../udcs/dwc3.N
> 
> The only thing the poor user needs to know is that *.0 is the first
> controller, *.1 is the second controller and so on.

But only once the directories are already there. But they need
to be created before.

AP


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