Re: [patch] nfs: use GFP_NOFS preloads for radix-tree insertion

[Nick Piggin <npiggin@suse.de>]

On Wed, Nov 14, 2007 at 10:06:27AM +0100, Peter Zijlstra wrote:
> 
> On Wed, 2007-11-14 at 05:20 +0100, Nick Piggin wrote:
> > On Tue, Nov 13, 2007 at 11:55:45AM +0100, Peter Zijlstra wrote:
> > > 
> > > On Thu, 2007-11-08 at 07:56 +0100, Nick Piggin wrote:
> > > > Here is the NFS version. I guess Trond should ack it before you pick it
> > > > up.
> > > > 
> > > > --
> > > > 
> > > > NFS should use GFP_NOFS mode radix tree preloads rather than GFP_ATOMIC
> > > > allocations at radix-tree insertion-time. This is important to reduce the
> > > > atomic memory requirement.
> > > 
> > > In another mail you said:
> > > 
> > > > Anyway we can also simplify the code because the insertion can't fail with a
> > > > preload.
> > > 
> > > Can we please avoid adding strict dependencies on that as the preload
> > > API is unsupportable in -rt.
> > 
> > You can surely support it. You just have to do per-thread preloads if you
> > want preemption left on.
> 
> Well, true, but that would mean adding stuff to task_struct, not the end
> of the world I guess.
> 
> But as it is leaving the error handling on each individual
> radix_tree_insert() allows us to just use GFP_KERNEL for everything.

Hmm, then you reintroduce the lock ordering which I got rid of. Not that
it's a particularly big deal in this case. I don't think you should noop
fundamental things like this just because they turn preempt off. At any
rate, it's not something that mainline can really be concerned with...


> The other, nicer option, is to do preload on the radix_tree_context
> object instead.

I don't know if you'd call it nicer... at least, not as nice as what's
upstream. So it seems like you'd have to have a custom solution anyway,
given that per-cpu preloads are probably the best we can do upstream.

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