Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver #17]

[Jeff Layton]

On Tue, 2020-03-03 at 09:55 -0700, Jens Axboe wrote:
> On 3/3/20 9:51 AM, Jeff Layton wrote:
> > On Tue, 2020-03-03 at 08:44 -0700, Jens Axboe wrote:
> > > On 3/3/20 7:24 AM, Greg Kroah-Hartman wrote:
> > > > On Tue, Mar 03, 2020 at 03:13:26PM +0100, Jann Horn wrote:
> > > > > On Tue, Mar 3, 2020 at 3:10 PM Greg Kroah-Hartman
> > > > > <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > > > > > On Tue, Mar 03, 2020 at 02:43:16PM +0100, Greg Kroah-Hartman wrote:
> > > > > > > On Tue, Mar 03, 2020 at 02:34:42PM +0100, Miklos Szeredi wrote:
> > > > > > > > On Tue, Mar 3, 2020 at 2:14 PM Greg Kroah-Hartman
> > > > > > > > <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > > > > > > > 
> > > > > > > > > > Unlimited beers for a 21-line kernel patch?  Sign me up!
> > > > > > > > > > 
> > > > > > > > > > Totally untested, barely compiled patch below.
> > > > > > > > > 
> > > > > > > > > Ok, that didn't even build, let me try this for real now...
> > > > > > > > 
> > > > > > > > Some comments on the interface:
> > > > > > > 
> > > > > > > Ok, hey, let's do this proper :)
> > > > > > 
> > > > > > Alright, how about this patch.
> > > > > > 
> > > > > > Actually tested with some simple sysfs files.
> > > > > > 
> > > > > > If people don't strongly object, I'll add "real" tests to it, hook it up
> > > > > > to all arches, write a manpage, and all the fun fluff a new syscall
> > > > > > deserves and submit it "for real".
> > > > > 
> > > > > Just FYI, io_uring is moving towards the same kind of thing... IIRC
> > > > > you can already use it to batch a bunch of open() calls, then batch a
> > > > > bunch of read() calls on all the new fds and close them at the same
> > > > > time. And I think they're planning to add support for doing
> > > > > open()+read()+close() all in one go, too, except that it's a bit
> > > > > complicated because passing forward the file descriptor in a generic
> > > > > way is a bit complicated.
> > > > 
> > > > It is complicated, I wouldn't recommend using io_ring for reading a
> > > > bunch of procfs or sysfs files, that feels like a ton of overkill with
> > > > too much setup/teardown to make it worth while.
> > > > 
> > > > But maybe not, will have to watch and see how it goes.
> > > 
> > > It really isn't, and I too thinks it makes more sense than having a
> > > system call just for the explicit purpose of open/read/close. As Jann
> > > said, you can't currently do a linked sequence of open/read/close,
> > > because the fd passing between them isn't done. But that will come in
> > > the future. If the use case is "a bunch of files", then you could
> > > trivially do "open bunch", "read bunch", "close bunch" in three separate
> > > steps.
> > > 
> > > Curious what the use case is for this that warrants a special system
> > > call?
> > > 
> > 
> > Agreed. I'd really rather see something more general-purpose than the
> > proposed readfile(). At least with NFS and SMB, you can compound
> > together fairly arbitrary sorts of operations, and it'd be nice to be
> > able to pattern calls into the kernel for those sorts of uses.
> > 
> > So, NFSv4 has the concept of a current_stateid that is maintained by the
> > server. So basically you can do all this (e.g.) in a single compound:
> > 
> > open <some filehandle get a stateid>
> > write <using that stateid>
> > close <same stateid>
> > 
> > It'd be nice to be able to do something similar with io_uring. Make it
> > so that when you do an open, you set the "current fd" inside the
> > kernel's context, and then be able to issue io_uring requests that
> > specify a magic "fd" value that use it.
> > 
> > That would be a really useful pattern.
> 
> For io_uring, you can link requests that you submit into a chain. Each
> link in the chain is done in sequence. Which means that you could do:
> 
> <open some file><read from that file><close that file>
> 
> in a single sequence. The only thing that is missing right now is a way
> to have the return of that open propagated to the 'fd' of the read and
> close, and it's actually one of the topics to discuss at LSFMM next
> month.
> 
> One approach would be to use BPF to handle this passing, another
> suggestion has been to have the read/close specify some magic 'fd' value
> that just means "inherit fd from result of previous". The latter sounds
> very close to the stateid you mention above, and the upside here is that
> it wouldn't explode the necessary toolchain to need to include BPF.
> 
> In other words, this is really close to being reality and practically
> feasible.
> 

Excellent.

Yes, the latter is exactly what I had in mind for this. I suspect that
that would cover a large fraction of the potential use-cases for this.

Basically, all you'd need to do is keep a pointer to struct file in the
internal state for the chain. Then, allow userland to specify some magic
fd value for subsequent chained operations that says to use that instead
of consulting the fdtable. Maybe use -4096 (-MAX_ERRNO - 1)?

That would cover the smb or nfs server sort of use cases, I think. For
the sysfs cases, I guess you'd need to dispatch several chains, but that
doesn't sound _too_ onerous.

In fact, with that you should even be able to emulate the proposed
readlink syscall in a userland library.
-- 
Jeff Layton <jlayton@xxxxxxxxxx>