Re: [PATCH 2/8] mm: De-indent struct page

[Matthew Wilcox <willy@infradead.org>]

On Mon, Dec 18, 2017 at 04:36:52PM +0100, Michal Hocko wrote:
> On Sat 16-12-17 08:44:19, Matthew Wilcox wrote:
> > From: Matthew Wilcox <mawilcox@microsoft.com>
> > 
> > I found the struct { union { struct { union { struct { } } } } }
> > layout rather confusing.  Fortunately, there is an easier way to write
> > this.  The innermost union is of four things which are the size of an
> > int, so the ones which are used by slab/slob/slub can be pulled up
> > two levels to be in the outermost union with 'counters'.  That leaves
> > us with struct { union { struct { atomic_t; atomic_t; } } } which
> > has the same layout, but is easier to read.
> 
> This is where the pahole output would be really helpeful. The patch
> looks OK, I will double check with a fresh brain tomorrow (with the rest
> of the series), though.

I got Arnaldo to change the pahole output to make this easier.  Here's
the result:

@@ -11,17 +11,15 @@
 	};                                               /*    16     8 */
 	union {
 		long unsigned int  counters;             /*    24     8 */
+		unsigned int       active;               /*    24     4 */
 		struct {
-			union {
-				atomic_t _mapcount;      /*    24     4 */
-				unsigned int active;     /*    24     4 */
-				struct {
-					unsigned int inuse:16; /*    24:16  4 */
-					unsigned int objects:15; /*    24: 1  4 */
-					unsigned int frozen:1; /*    24: 0  4 */
-				};                       /*    24     4 */
-				int units;               /*    24     4 */
-			};                               /*    24     4 */
+			unsigned int inuse:16;           /*    24:16  4 */
+			unsigned int objects:15;         /*    24: 1  4 */
+			unsigned int frozen:1;           /*    24: 0  4 */
+		};                                       /*    24     4 */
+		int                units;                /*    24     4 */
+		struct {
+			atomic_t   _mapcount;            /*    24     4 */
 			atomic_t   _refcount;            /*    28     4 */
 		};                                       /*    24     8 */
 	};                                               /*    24     8 */


It's even more dramatic if you use diff -uw (ignore whitespace):

@@ -11,9 +11,6 @@
 	};                                               /*    16     8 */
 	union {
 		long unsigned int  counters;             /*    24     8 */
-		struct {
-			union {
-				atomic_t _mapcount;      /*    24     4 */
 				unsigned int active;     /*    24     4 */
 				struct {
 					unsigned int inuse:16; /*    24:16  4 */
@@ -21,7 +18,8 @@
 					unsigned int frozen:1; /*    24: 0  4 */
 				};                       /*    24     4 */
 				int units;               /*    24     4 */
-			};                               /*    24     4 */
+		struct {
+			atomic_t   _mapcount;            /*    24     4 */
 			atomic_t   _refcount;            /*    28     4 */
 		};                                       /*    24     8 */
 	};                                               /*    24     8 */


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