RE: [PATCH] zsmalloc: do not use bit_spin_lock

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Vitaly Wool [mailto:vitaly.wool@xxxxxxxxxxxx]
> Sent: Tuesday, December 22, 2020 10:44 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>
> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>; Minchan Kim <minchan@xxxxxxxxxx>; Mike
> Galbraith <efault@xxxxxx>; LKML <linux-kernel@xxxxxxxxxxxxxxx>; linux-mm
> <linux-mm@xxxxxxxxx>; Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>;
> NitinGupta <ngupta@xxxxxxxxxx>; Sergey Senozhatsky
> <sergey.senozhatsky.work@xxxxxxxxx>; Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx>; tiantao (H) <tiantao6@xxxxxxxxxxxxx>
> Subject: Re: [PATCH] zsmalloc: do not use bit_spin_lock
> 
> On Tue, 22 Dec 2020, 03:11 Song Bao Hua (Barry Song),
> <song.bao.hua@xxxxxxxxxxxxx> wrote:
> >
> >
> >
> > > -----Original Message-----
> > > From: Song Bao Hua (Barry Song)
> > > Sent: Tuesday, December 22, 2020 3:03 PM
> > > To: 'Vitaly Wool' <vitaly.wool@xxxxxxxxxxxx>
> > > Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>; Minchan Kim <minchan@xxxxxxxxxx>;
> Mike
> > > Galbraith <efault@xxxxxx>; LKML <linux-kernel@xxxxxxxxxxxxxxx>; linux-mm
> > > <linux-mm@xxxxxxxxx>; Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>;
> > > NitinGupta <ngupta@xxxxxxxxxx>; Sergey Senozhatsky
> > > <sergey.senozhatsky.work@xxxxxxxxx>; Andrew Morton
> > > <akpm@xxxxxxxxxxxxxxxxxxxx>; tiantao (H) <tiantao6@xxxxxxxxxxxxx>
> > > Subject: RE: [PATCH] zsmalloc: do not use bit_spin_lock
> > >
> > >
> > > > I'm still not convinced. Will kmap what, src? At this point src might
> become
> > > just a bogus pointer.
> > >
> > > As long as the memory is still there, we can kmap it by its page struct.
> But
> > > if
> > > it is not there anymore, we have no way.
> > >
> > > > Why couldn't the object have been moved somewhere else (due to the compaction
> > > mechanism for instance)
> > > > at the time DMA kicks in?
> > >
> > > So zs_map_object() will guarantee the src won't be moved by holding those
> > > preemption-disabled lock?
> > > If so, it seems we have to drop the MOVABLE gfp in zswap for zsmalloc case?
> > >
> >
> > Or we can do get_page() to avoid the movement of the page.
> 
> 
> I would like to discuss this more in zswap context than zsmalloc's.
> Since zsmalloc does not implement reclaim callback, using it in zswap
> is a corner case anyway.

I see. But it seems we still need a solution for the compatibility
of zsmalloc and zswap? this will require change in either zsmalloc
or zswap. 
or do you want to make zswap depend on !ZSMALLOC?

> 
> zswap, on the other hand, may be dealing with some new backends in
> future which have more chances to become mainstream. Imagine typical
> NUMA-like cases, i. e. a zswap pool allocated in some kind SRAM, or in
> unused video memory. In such a case if you try to use a pointer to an
> invalidated zpool mapping, you are on the way to thrash the system.
> So: no assumptions that the zswap pool is in regular linear RAM should
> be made.
> 
> ~Vitaly

Thanks
Barry