Re: [patch] radix-tree: avoid atomic allocations for preloaded insertions

[Nick Piggin <npiggin@suse.de>]

On Wed, Nov 07, 2007 at 09:02:04PM -0800, Andrew Morton wrote:
> > On Thu, 8 Nov 2007 05:54:04 +0100 Nick Piggin <npiggin@suse.de> wrote:
> > > - doesn't solve the problem which it claims to be solving
> > >   (radix_tree_insert() doesn't deplete atomic reserves as long as the
> > >   caller uses radix_tree_preload(GFP_KERNEL))
> > 
> > I'm pretty sure it does. I don't follow exactly why you say it doesn't.
> >  
> 
> I was wrong.  The radix_tree_preloads will remain full and we'll just keep
> on doing GFP_ATOMIC allocations.

OK. Normally it's not a huge issue, because you've recently allocated a
page, but actually it can become a problem if: the page comes out of a
different zone than the radix tree node; or you're doing something like
mpage_readpages code that can do a lot of work between allocating the
pages and inserting them.

 
> > > - is probably desirable as a simplify-the-locking-hierarchy thing, but a)
> > >   should be presented as such and
> > 
> > It's primarily to avoid GFP_ATOMIC allocations. Simplify the locking
> > hierarcy is secondary and I put that in the changelog.
> > 
> > 
> > > b) needs code comments explaining why it
> > >   is correct and needs a big fat TODO explaining how we should get that
> > >   kmem_cache_alloc() out of there, an how we should do it.
> > > 
> > > OK?
> > 
> > I don't really know about getting that kmem_cache_alloc out of there.
> > For radix trees that are protected by sleeping locks, you don't actually
> > need to disable preempt and you can do sleeping allocations there.
> 
> If the radix tree's gfp_mask is GFP_ATOMIC, radix_tree_insert() can require
> that the preloads be full.

So we can put that invariant check in radix_tree_insert(), and I could
refactor / comment the radix_tree_node_alloc a bit so that it is clearer?

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