Re: [PATCHv4 01/17] zram: switch to non-atomic entry locking

[Sergey Senozhatsky]

On (25/02/03 12:21), Sergey Senozhatsky wrote:
> On (25/01/31 19:41), Hillf Danton wrote:
> > On Fri, 31 Jan 2025 18:06:00 +0900 Sergey Senozhatsky
> > > Concurrent modifications of meta table entries is now handled
> > > by per-entry spin-lock.  This has a number of shortcomings.
> > > 
> > > First, this imposes atomic requirements on compression backends.
> > > zram can call both zcomp_compress() and zcomp_decompress() under
> > > entry spin-lock, which implies that we can use only compression
> > > algorithms that don't schedule/sleep/wait during compression and
> > > decompression.  This, for instance, makes it impossible to use
> > > some of the ASYNC compression algorithms (H/W compression, etc.)
> > > implementations.
> > > 
> > > Second, this can potentially trigger watchdogs.  For example,
> > > entry re-compression with secondary algorithms is performed
> > > under entry spin-lock.  Given that we chain secondary
> > > compression algorithms and that some of them can be configured
> > > for best compression ratio (and worst compression speed) zram
> > > can stay under spin-lock for quite some time.
> > > 
> > > Do not use per-entry spin-locks and instead convert it to an
> > > atomic_t variable which open codes reader-writer type of lock.
> > > This permits preemption from slot_lock section, also reduces
> > > the sizeof() zram entry when lockdep is enabled.
> > > 
> > Nope, the price of cut in size will be paid by extra hours in debugging,
> > given nothing is free.
> 
> This has been a bit-spin-lock basically forever, until late last
> year when it was switched to a spinlock, for reasons unrelated
> to debugging (as far as I understand it).  See 9518e5bfaae19 (zram:
> Replace bit spinlocks with a spinlock_t).

Just want to clarify a little:

That "also reduces sizeof()" thing was added last minute (I think before
sending v4 out) and it was not an intention of this patch.  I just recalled
that sizeof() zram entry under lockdep was brought by linux-rt folks when
they discussed the patch that converter zram entry bit-spinlock into a
spinlock, and then I just put that line.

> > > -static void zram_slot_unlock(struct zram *zram, u32 index)
> > > +static void zram_slot_read_unlock(struct zram *zram, u32 index)
> > >  {
> > > -	spin_unlock(&zram->table[index].lock);
> > > +	atomic_dec(&zram->table[index].lock);
> > >  }
> > >  
> > Given no boundaries of locking section marked in addition to lockdep, 
> > this is another usual case of inventing lock in 2025.
> 
> So zram entry has been memory-saving driver, pretty much always, and not
> debug-ability driver, I'm afraid.

Before zram per-entry bit-spinlock there was a zram table rwlock, that
protected all zram meta table entries.  And before that there was a
per-zram device rwsem that synchronized all operation and protected
the entire zram meta table, so zram was fully preemptible back then.
Kind of interesting, that's what I want it to be now again.