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Date: Tue, 10 Apr 2018 13:57:57 -0700
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH] slub: Remove use of page->counter
Message-ID: <20180410205757.GD21336@bombadil.infradead.org>
References: <20180410195429.GB21336@bombadil.infradead.org>
 <alpine.DEB.2.20.1804101545350.30437@nuc-kabylake>
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To: Christopher Lameter <cl@linux.com>
Cc: linux-mm@kvack.org

On Tue, Apr 10, 2018 at 03:47:28PM -0500, Christopher Lameter wrote:
> On Tue, 10 Apr 2018, Matthew Wilcox wrote:
> 
> > In my continued attempt to clean up struct page, I've got to the point
> > where it'd be really nice to get rid of 'counters'.  I like the patch
> > below because it makes it clear when & where we're doing "weird" things
> > to access the various counters.
> 
> Well sounds good.
> 
> > struct {
> > 	unsigned long flags;
> > 	union {
> > 		struct {
> > 			struct address_space *mapping;
> > 			pgoff_t index;
> > 		};
> > 		struct {
> > 			void *s_mem;
			/* Dword boundary */
> > 			void *freelist;
> > 		};
> > 		...
> > 	};
> > 	union {
> > 		atomic_t _mapcount;
> > 		unsigned int active;
> 
> Is this aligned on a doubleword boundary? Maybe move the refcount below
> the flags field?

You need freelist and _mapcount to be in the same dword.  There's no
space to put them both in dword 0, so that's used for flags and mapping
/ s_mem.  Then freelist, mapcount and refcount are in dword 1 (on 64-bit),
or freelist & mapcount are in dword 1 on 32-bit.  After that, 32 and 64-bit
no longer line up on the same dword boundaries.