linux-kernel - Re: mm: ptl is not bloated if it fits in pointer

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Message-ID: <20131216100446.GT21999@twins.programming.kicks-ass.net>
Date:	Mon, 16 Dec 2013 11:04:46 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: mm: ptl is not bloated if it fits in pointer

On Mon, Dec 16, 2013 at 01:04:13AM -0800, Hugh Dickins wrote:
> It's silly to force the 64-bit CONFIG_GENERIC_LOCKBREAK architectures
> to kmalloc eight bytes for an indirect page table lock: the lock needs
> to fit in the space that a pointer to it would occupy, not into an int.

Ah, no. A spinlock is very much assumed to be 32bit, any spinlock that's
bigger than that is bloated.

For the page-frame case we do indeed not care about the strict 32bit but
more about not being larger than a pointer, however there are already
other users.

See for instance include/linux/lockref.h and lib/lockref.c, they very
much require the spinlock to be 32bit and the below would break that.
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