Re: [PATCH] zsmalloc: do not use bit_spin_lock

[Vitaly Wool <vitaly.wool@konsulko.com>]

On Tue, 22 Dec 2020, 22:06 Song Bao Hua (Barry Song),
<song.bao.hua@hisilicon.com> wrote:
>
>
>
> > -----Original Message-----
> > From: Vitaly Wool [mailto:vitaly.wool@konsulko.com]
> > Sent: Tuesday, December 22, 2020 10:44 PM
> > To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>
> > Cc: Shakeel Butt <shakeelb@google.com>; Minchan Kim <minchan@kernel.org>; Mike
> > Galbraith <efault@gmx.de>; LKML <linux-kernel@vger.kernel.org>; linux-mm
> > <linux-mm@kvack.org>; Sebastian Andrzej Siewior <bigeasy@linutronix.de>;
> > NitinGupta <ngupta@vflare.org>; Sergey Senozhatsky
> > <sergey.senozhatsky.work@gmail.com>; Andrew Morton
> > <akpm@linux-foundation.org>; tiantao (H) <tiantao6@hisilicon.com>
> > 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@hisilicon.com> wrote:
> > >
> > >
> > >
> > > > -----Original Message-----
> > > > From: Song Bao Hua (Barry Song)
> > > > Sent: Tuesday, December 22, 2020 3:03 PM
> > > > To: 'Vitaly Wool' <vitaly.wool@konsulko.com>
> > > > Cc: Shakeel Butt <shakeelb@google.com>; Minchan Kim <minchan@kernel.org>;
> > Mike
> > > > Galbraith <efault@gmx.de>; LKML <linux-kernel@vger.kernel.org>; linux-mm
> > > > <linux-mm@kvack.org>; Sebastian Andrzej Siewior <bigeasy@linutronix.de>;
> > > > NitinGupta <ngupta@vflare.org>; Sergey Senozhatsky
> > > > <sergey.senozhatsky.work@gmail.com>; Andrew Morton
> > > > <akpm@linux-foundation.org>; tiantao (H) <tiantao6@hisilicon.com>
> > > > 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?

No, I really don't think we should go that far. What if we add a flag
to zpool, named like "can_sleep_mapped", and have it set for
zbud/z3fold?
Then zswap could go the current path if the flag is set; and if it's
not set, and mutex_trylock fails, copy data from src to a temporary
buffer, then unmap the handle, take the mutex, process the buffer
instead of src. Not the nicest thing to do but at least it won't break
anything.

~Vitaly

> > 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