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

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Message-ID: <ZsebVzVVzcA4QBhG@finisterre.sirena.org.uk>
Date: Thu, 22 Aug 2024 21:45:35 +0100
From: Mark Brown <broonie@...nel.org>
To: Matthew Wilcox <willy@...radead.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 08:55:20PM +0100, Matthew Wilcox wrote:
> On Thu, Aug 22, 2024 at 08:48:56PM +0100, Mark Brown wrote:

> > 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

> 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.

If the external lock guarantees that nothing can possibly be contending
access to the tree (including the read side) I don't see any issue
there?

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