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Message-ID: <20161024160122.GA2125@cmpxchg.org>
Date: Mon, 24 Oct 2016 12:01:22 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Dave Chinner <david@...morbit.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Jan Kara <jack@...e.cz>, Dave Jones <davej@...emonkey.org.uk>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
kernel-team@...com
Subject: Re: [PATCH 3/5] lib: radix-tree: native accounting and tracking of
special entries
Hi Dave,
On Fri, Oct 21, 2016 at 09:33:08AM +1100, Dave Chinner wrote:
> On Wed, Oct 19, 2016 at 01:24:26PM -0400, Johannes Weiner wrote:
> > With this in place, the next patch can restore refault detection in
> > single-page files. It will also move the shadow count from the upper
> > bits of count to the new special counter, and then shrink count to a
> > char as well; the growth of struct radix_tree_node is temporary.
[...]
> > @@ -90,9 +91,10 @@ enum radix_tree_tags {
> > #define RADIX_TREE_COUNT_MASK ((1UL << RADIX_TREE_COUNT_SHIFT) - 1)
> >
> > struct radix_tree_node {
> > - unsigned char shift; /* Bits remaining in each slot */
> > - unsigned char offset; /* Slot offset in parent */
> > - unsigned int count;
> > + unsigned char shift; /* Bits remaining in each slot */
> > + unsigned char offset; /* Slot offset in parent */
> > + unsigned int count; /* Total entry count */
> > + unsigned char special; /* Special entry count */
>
> How about putting the new char field into the implicit hole between
> offset and count? pahole is your friend here:
>
> struct radix_tree_node {
> unsigned char shift; /* 0 1 */
> unsigned char offset; /* 1 1 */
>
> /* XXX 2 bytes hole, try to pack */
>
> unsigned int count; /* 4 4 */
> .....
The next patch turns `count' into an unsigned char again, so the hole
is only temporary.
Thanks
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