Re: [PATCH v10 01/19] zram: sleepable entry locking

[Sergey Senozhatsky]

On (26/07/07 08:40), Sebastian Andrzej Siewior wrote:
[..]
> >  static void zram_slot_lock(struct zram *zram, u32 index)
> >  {
> > -	spin_lock(&zram->table[index].lock);
> > +	unsigned long *lock = &zram->table[index].flags;
> > +
> > +	mutex_acquire(slot_dep_map(zram, index), 0, 0, _RET_IP_);
> > +	wait_on_bit_lock(lock, ZRAM_ENTRY_LOCK, TASK_UNINTERRUPTIBLE);
> > +	lock_acquired(slot_dep_map(zram, index), _RET_IP_);
> >  }
> 
> Looking at this, is there a special need to have a lockdep map per
> table? Wouldn't it be enough to have one per zram? Logically you use the
> same __key so I don't think it makes much of a difference but you could
> lower the memory usage a bit by having less of those structs per zram.

So the idea was to separate lockdep maps and keep them per-device
because various zram devices can be on different IO paths with very
different locking graphs.  E.g. zram0 can be a swap device with all
the locks that swap layer has, while zram1 can be a "normal" block
device mounted with ext4 with all the locks that ext4/vfs bring.