Re: [PATCH] zsmalloc: do not use bit_spin_lock

[Vitaly Wool]

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.

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
>
>
> > >
> > > >
> > > > ~Vitaly
> > >
> >
> > Thanks
> > Barry
>
>