Re: [RFC 3/3] SGI Altix cross partition memory (XPMEM)

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 22 Aug 2007 12:00:11 -0500
Dean Nelson <dcn@sgi.com> wrote:

> 
>   3) WARNING: declaring multiple variables together should be avoided
> 
> checkpatch.pl is erroneously commplaining about the following found in five
> different functions in arch/ia64/sn/kernel/xpmem_pfn.c.
> 
> 	int n_pgs = xpmem_num_of_pages(vaddr, size);

What warning does it generate here?

> > - xpmem_fault_handler() appears to have imposed a kernel-wide rule that
> >   when taking multiple mmap_sems, one should take the lowest-addressed one
> >   first?  If so, that probably wants a mention in that locking comment in
> >   filemap.c
> 
> Sure. After looking at the lock ordering comment block in mm/filemap.c, it
> wasn't clear to me how best to document this. Any suggestions/help would
> be most appreciated.

umm,

 * when taking multiple mmap_sems, one should take the lowest-addressed one
 * first

 ;)

> > - xpmem_fault_handler() does atomic_dec(&seg_tg->mm->mm_users).  What
> >   happens if that was the last reference?
> 
> When /dev/xpmem is opened by a user process, xpmem_open() incs mm_users
> and when it is flushed, xpmem_flush() decs it (via mmput()) after having
> ensured that no XPMEM attachments exist of this mm. Thus the dec in
> xpmem_fault_handler() will never take it to 0.

OK.  Generally if a reviewer asks a question like this, it indicates that a
code comment is needed.  Because it is likely that others will later wonder
the same thing.

> > - Has it all been tested with lockdep enabled?  Jugding from all the use
> >   of SPIN_LOCK_UNLOCKED, it has not.
> >
> >   Oh, ia64 doesn't implement lockdep.  For this code, that is deeply
> >   regrettable.
> 
> No, it hasn't been tested with lockdep. But I have switched it from using
> SPIN_LOCK_UNLOCKED to spin_lock_init().
> 
> > ! This code all predates the nopage->fault conversion and won't work in
> >   current kernels.
> 
> I've switched from using nopage to using fault. I read that it is intended
> that nopfn also goes away. If this is the case, then the BUG_ON if VM_PFNMAP
> is set would make __do_fault() a rather unfriendly replacement for do_no_pfn().
> 
> > - xpmem_attach() does smp_processor_id() in preemptible code.  Lucky that
> >   ia64 doesn't do preempt?
> 
> Actually, the code is fine as is even with preemption configured on. All it's
> doing is ensuring that the thread was previously pinned to the CPU it's
> currently running on. If it is, it can't be moved to another CPU via
> preemption, and if it isn't, the check will fail and we'll return -EINVAL
> and all is well.

OK.  Running smp_processor_id() from within preemptible code will generate
a warning, but the code is sneaky enough to prevent that warning if the
calling task happens to be pinned to a single CPU.