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Date: Mon, 16 Dec 2013 11:04:46 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Hugh Dickins <hughd@google.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: mm: ptl is not bloated if it fits in pointer
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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.