[PATCH/RFC] staging/lustre: Rework class_process_proc_param

[Greg Kroah-Hartman]

On Sun, Mar 19, 2017 at 12:41:20AM -0400, Oleg Drokin wrote:
> 
> On Mar 19, 2017, at 12:29 AM, Greg Kroah-Hartman wrote:
> 
> > On Sat, Mar 18, 2017 at 11:17:55AM -0400, Oleg Drokin wrote:
> >> 
> >> On Mar 18, 2017, at 6:34 AM, Greg Kroah-Hartman wrote:
> >> 
> >>> On Sat, Mar 18, 2017 at 02:24:08AM -0400, Oleg Drokin wrote:
> >>>> Ever since sysfs migration, class_process_proc_param stopped working
> >>>> correctly as all the useful params were no longer present as lvars.
> >>>> Replace all the nasty fake proc writes with hopefully less nasty
> >>>> kobject attribute search and then update the attributes as needed.
> >>>> 
> >>>> Signed-off-by: Oleg Drokin <green at linuxhacker.ru>
> >>>> Reported-by: Al Viro <viro at ZenIV.linux.org.uk>
> >>>> ---
> >>>> Al has quite rightfully complained in the past that class_process_proc_param
> >>>> is a terrible piece of code and needs to go.
> >>>> This patch is an attempt at improving it somewhat and in process drop
> >>>> all the user/kernel address space games we needed to play to make it work
> >>>> in the past (and which I suspect attracted Al's attention in the first place).
> >>>> 
> >>>> Now I wonder if iterating kobject attributes like that would be ok with
> >>>> you Greg, or do you think there is a better way?
> >>>> class_find_write_attr could be turned into something generic since it's
> >>>> certainly convenient to reuse same table of name-write_method pairs,
> >>>> but I did some cursory research and nobody else seems to need anything
> >>>> of the sort in-tree.
> >>>> 
> >>>> I know ll_process_config is still awful and I will likely just
> >>>> replace the current hack with kset_find_obj, but I just wanted to make
> >>>> sure this new approach would be ok before spending too much time on it.
> >>> 
> >>> I'm not quite sure what exactly you are even trying to do here.  What is
> >>> this interface?  Who calls it, and how?  What does it want to do?
> >> 
> >> This is a configuration client code.
> >> Management server has ability to pass down config information in the form of:
> >> fsname.subsystem.attribute=value to clients and other servers
> >> (subsystem determines if it's something of interest of clients or servers or
> >> both).
> >> This could be changed in the real time - i.e. you update it on the server and
> >> that gets propagated to all the clients/servers, so no need to ssh into
> >> every node to manually apply those changes and worry about client restarts
> >> (the config is remembered at the management server and would be applied to any
> >> new nodes connecting/across server restarts and such).
> >> 
> >> So the way it then works then is once the string fsname.subsystem.attribute=value is delivered to the client, we find all instances of filesystem with fsname and then
> >> all subsystems within it (one kobject per subsystem instance) and then update the
> >> attributes to the value supplied.
> >> 
> >> The same filesystem might be mounted more than once and then some layers might have
> >> multiple instances inside a single filesystems.
> >> 
> >> In the end it would turn something like lustre.osc.max_dirty_mb=128 into
> >> writes to
> >> /sys/fs/lustre/osc/lustre-OST0000-osc-ffff8800d75ca000/max_dirty_mb and
> >> /sys/fs/lustre/osc/lustre-OST0001-osc-ffff8800d75ca000/max_dirty_mb
> >> without actually iterating in sysfs namespace.
> > 
> > Wait, who is doing the "write"?  From within the kernel?  Or some
> > userspace app?  I'm guessing from within the kernel, you are receiving
> > the data from some other transport within the filesystem and then need
> > to apply the settings?
> 
> Yes, kernel code gets the notification "hey, there's a config change, come get it",
> then it requests the diff in the config and does the "write' by updating the
> attributes.
> 
> >> The alternative we considered is we can probably just do an upcall and have
> >> a userspace tool called with the parameter verbatim and try to figure it out,
> >> but that seems a lot less ideal, and also we'll get a bunch of complications from
> >> containers and such too, I imagine.
> > 
> > Yeah, no, don't do an upcall, that's a mess.
> > 
> >> The function pre-this patch is assuming that all these values are part of
> >> a list of procfs values (no longer true after sysfs migration) so just iterates
> >> that list and calls the write for matched names (but also needs to supply a userspace
> >> buffer so looks much uglier too).
> > 
> > For kobjects, you don't need userspace buffers, so this should be
> > easier.
> 
> Right, it's less of a mess due to that.
> 
> >> Hopefully this makes at least some sense.
> > 
> > Yes, a bit more, but ugh, what a mess :)
> 
> Overall big systems are quite messy in many ways no matter what you do,
> so it's always a case of pick your poison.

Oh I know, I wasn't complaining, it's just a fact of life :)

> >>> You can look up attributes for a kobject easily in the show/store
> >>> functions (and some drivers just have a generic one and then you look at
> >>> the string to see which attribute you are wanting to reference.)  But
> >>> you seem to be working backwards here, why do you have to look up a
> >>> kobject?
> >> 
> >> But that leads to the need to list attribute names essentially twice:
> >> once for the attributes list, once in the show/set function to figure
> >> out how to deal with that name.
> > 
> > Yes, you don't need/want to do that.
> > 
> > Let me go review the code now with that info, thanks, it makes more
> > sense.
> 
> Thanks!
> In the end if the approach is mostly acceptable, I imagine if we just register the
> kobject for the config changes, we can do away with the individual layers calling the
> class_process_attr_param and instead have the class_process_config() call it directly
> more or less.

Yes, that seems like it would work as well.

thanks,

greg k-h