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

Andrew Morton
2007-08-22T18:05:56Z

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.