Re: [PATCH 1/3] zram: use u32 for entry ac_time tracking

[Sergey Senozhatsky]

On (25/12/15 17:31), Brian Geffon wrote:
[..]
> >  struct zram_table_entry {
> >         unsigned long handle;
> > -       unsigned long flags;
> > +       union {
> > +               unsigned long __lock;
> > +               struct attr {
> > +                       u32 flags;
> >  #ifdef CONFIG_ZRAM_TRACK_ENTRY_ACTIME
> > -       ktime_t ac_time;
> > +                       u32 ac_time;
> >  #endif
> 
> Why not just always enable CONFIG_ZRAM_TRACK_ENTRY_ACTIME now that it
> doesn't consume any additional space?

It's "free" only on x64.  On 32bit systems the removal of
ZRAM_TRACK_ENTRY_ACTIME will unconditionally add 4 bytes
per zram_table_entry.

> Also, why can't we do this with a single unsigned long flags
> as before and have a simple method that isolates and casts the
> lower 32bits as a u32?

There are no upper and lower 32 bits in unsigned long on 32bit systems.