On Tue, Jul 24, 2018 at 07:55:25PM -0700, Matthew Wilcox wrote:
> On Wed, Jul 25, 2018 at 11:51:06AM +0900, Minchan Kim wrote:
> > On Tue, Jul 24, 2018 at 07:35:32PM -0700, Matthew Wilcox wrote:
> > > There is NOTHING in a union with _refcount.
> >
> > Confusing. Matthew, what am I missing?
> >
> > Before:
> >
> > counters consumes 8 bytes
> > units and _refcount consumes each 4 bytes so memory space is not overlapped.
>
> Correct. _mapcount is at offset 24, as is units. _refcount is at offset 28.
>
> > union {
> > unsigned long counters;
> > struct {
> > union {
> > atomic_t _mapcount;
> > int units; /* SLOB */
> > };
> > atomic_t _refcount;
> > };
> > };
> >
> >
> > After:
> >
> > Now, units is overlapped with _refcount and _mapcount.
>
> No. units is at offset 24, as is _mapcount. But _refcount is still at
> offset 28.
Ah, got it what you mean. Sorry for the noise, Matthew.
>
> > union {
> > unsigned long counters;
> > int units; /* SLOB */
> > struct { /* Page cache */
> > atomic_t _mapcount;
> > atomic_t _refcount;
> > };
> > };
