[dm-devel] Re: [PATCH resubmit] do_mount: reduce stack consumption

[Neil Brown]

On Tuesday November 8, nickpiggin at yahoo.com.au wrote:
> Andrew Morton wrote:
> > 
> > More state in the task_strut is a bit sad, but not nearly as sad as deep
> > recursion in our deepest codepath..
> > 
> > Possibly one could do:
> > 
> > struct make_request_state {
> > 	struct bio *bio_list;
> > 	struct bio **bio_tail;
> > };
> > 
> > and stick a `struct make_request_state *' into the task_struct and actually
> > allocate the thing on the stack.  That's not much nicer though.
> 
> Possibly it could go into struct io_context?
> 

My quick reading of the code says that we could have to 
allocate the struct right there in generic_make_request, and I don't
think we can be certain that such an allocation will succeed.

Code that uses io_context can limp along if it doesn't exist.  
The new generic_make_request needs this bio_list to be present 
or it cannot do it's job.

Just how tight are we for space in task_struct?  It seems to have a
fair amount of cruft in it.
Is it getting close to one-page or something?
Can we just split the less interesting stuff up into a separate
structure, allocate a separate page for that are fork time, and leave 
just a pointer in the task_struct?

NeilBrown