On Mon 18-12-17 08:19:02, Matthew Wilcox wrote:
> 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 */
>
Excelent! Could you add the later one to the changelog please? With
that
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
I will go over the rest of the series tomorrow.
--
Michal Hocko
SUSE Labs
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