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Date: Fri, 20 Apr 2018 13:27:37 -0700
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v3 05/14] mm: Move 'private' union within struct page
Message-ID: <20180420202737.GE10788@bombadil.infradead.org>
References: <20180418184912.2851-1-willy@infradead.org>
 <20180418184912.2851-6-willy@infradead.org>
 <alpine.DEB.2.20.1804201024090.18006@nuc-kabylake>
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To: Christopher Lameter <cl@linux.com>
Cc: linux-mm@kvack.org, Matthew Wilcox <mawilcox@microsoft.com>, Andrew Morton <akpm@linux-foundation.org>, "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>, Lai Jiangshan <laijs@cn.fujitsu.com>, Pekka Enberg <penberg@kernel.org>, Vlastimil Babka <vbabka@suse.cz>

On Fri, Apr 20, 2018 at 10:25:14AM -0500, Christopher Lameter wrote:
> On Wed, 18 Apr 2018, Matthew Wilcox wrote:
> > +	union {
> > +		unsigned long private;
> > +#if USE_SPLIT_PTE_PTLOCKS
> > +#if ALLOC_SPLIT_PTLOCKS
> > +		spinlock_t *ptl;
> > +#else
> > +		spinlock_t ptl;
> 
> ^^^^ This used to be defined at the end of the struct so that you could
> have larger structs for spinlocks here (debugging and some such thing).
> 
> Could this not misalign the rest?

Nope:

include/linux/mm_types_task.h:#define ALLOC_SPLIT_PTLOCKS       (SPINLOCK_SIZE > BITS_PER_LONG/8)

So we'll only store a spinlock here if it's <= sizeof(long); otherwise
we'll store a pointer here.