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@xxxxxxxxxxxxx>
> >
> > 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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