Re: [LSF/MM] schedule suggestion

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2018-04-19 at 12:30 -0400, Jerome Glisse wrote:
> On Thu, Apr 19, 2018 at 07:43:56AM -0700, Matthew Wilcox wrote:
> > On Thu, Apr 19, 2018 at 10:38:25AM -0400, Jerome Glisse wrote:
> > > Oh can i get one more small slot for fs ? I want to ask if they are
> > > any people against having a callback everytime a struct file is added
> > > to a task_struct and also having a secondary array so that special
> > > file like device file can store something opaque per task_struct per
> > > struct file.
> > 
> > Do you really want something per _thread_, and not per _mm_?
> 
> Well per mm would be fine but i do not see how to make that happen with
> reasonable structure. So issue is that you can have multiple task with
> same mm but different file descriptors (or am i wrong here ?) thus there
> would be no easy way given a struct file to lookup the per mm struct.
> 
> So as a not perfect solution i see a new array in filedes which would
> allow device driver to store a pointer to their per mm data structure.
> To be fair usualy you will only have a single fd in a single task for
> a given device.
> 
> If you see an easy way to get a per mm per inode pointer store somewhere
> with easy lookup i am all ears :)
> 

I may be misunderstanding, but to be clear: struct files don't get
added to a thread, per-se.

When userland calls open() or similar, the struct file gets added to
the files_struct. Those are generally shared with other threads within
the same process. The files_struct can also be shared with other
processes if you clone() with the right flags.

Doing something per-thread on every open may be rather difficult to do.
-- 
Jeff Layton <jlayton@kernel.org>