linux-kernel - Re: [PATCH 1/5] maple_tree: Allow external locks to be configured with their map

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Message-ID: <ZseXqP6q7qyFeiCO@casper.infradead.org>
Date: Thu, 22 Aug 2024 20:55:20 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Mark Brown <broonie@...nel.org>
Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Cristian Ciocaltea <cristian.ciocaltea@...labora.com>,
	maple-tree@...ts.infradead.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] maple_tree: Allow external locks to be configured
 with their map

On Thu, Aug 22, 2024 at 08:48:56PM +0100, Mark Brown wrote:
> On Thu, Aug 22, 2024 at 08:21:40PM +0100, Matthew Wilcox wrote:
> > On Thu, Aug 22, 2024 at 08:13:35PM +0100, Mark Brown wrote:
> 
> > > Currently the maple tree code allows external locks to be configured by
> > > passing the lock itself. This is generally helpful and convenient but is
> 
> > No, it's a really bad idea.  Stop doing it.  Use the internal lock.
> > It's a temporary hack we put in and I'm really regretting allowing it.
> 
> I mean, we do use the internal lock here since otherwise lockdep moans
> but it's pure overhead which just complicates the code.  It's only ever
> taken within another lock, meaning it winds up protecting nothing for
> these maple trees.  We can't go the other way round and use the maple
> tree lock as the regmap lock since apart from anything else it's a spin
> lock and we need to use a mutex most of the time to support busses that
> sleep during I/O.

When it's an uncontended spinlock, there's really no overhead.  I wish I'd
been firmer on that point earlier and prohibited the external lock hack.

The point is that the lock protects the tree.  If we are ever going to
be able to defragment slabs (and I believe this is an ability that Linux
must gain), we must be able to go from the object (the maple node) to
a lock that will let us reallocate the node.  If there's some external
lock that protects the tree, we can't possibly do that.