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

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Message-Id: <20070822124928.19bf0431.akpm@linux-foundation.org>
Date:	Wed, 22 Aug 2007 12:49:28 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dean Nelson <dcn@....com>, Andy Whitcroft <apw@...dowen.org>
Cc:	linux-ia64@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, tony.luck@...el.com, jes@....com
Subject: Re: [RFC 3/3] SGI Altix cross partition memory (XPMEM)

On Wed, 22 Aug 2007 14:15:16 -0500
Dean Nelson <dcn@....com> wrote:

> On Wed, Aug 22, 2007 at 11:04:22AM -0700, Andrew Morton wrote:
> > On Wed, 22 Aug 2007 12:00:11 -0500
> > Dean Nelson <dcn@....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?
> 
> The WARNING #3 above "declaring multiple variables together should be avoided".
> There is only one variable being declared, which happens to be initialized by
> the function xpmem_num_of_pages().

Ah, I think I recall seeing a report of that earlier.  Maybe it's been fixed?

> ...
> > > 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.
> 
> Would it make more sense in this particular case to replace the call to
> smp_processor_id() in xpmem_attach() with a call to raw_smp_processor_id()
> instead, and add a comment explaining why?

Your call ;)  Either will be OK, I expect.
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