linux-kernel - Re: [PATCH] zsmalloc: do not use bit_spin

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Message-ID: <CAM4kBBJjCYX0DQZ8de9LsFV6L+eF4tZe-NN=jiAz9WLWYrsCsQ@mail.gmail.com>
Date:   Tue, 22 Dec 2020 10:44:12 +0100
From:   Vitaly Wool <vitaly.wool@...sulko.com>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Mike Galbraith <efault@....de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        NitinGupta <ngupta@...are.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "tiantao (H)" <tiantao6@...ilicon.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@...ilicon.com> wrote:
>
>
>
> > -----Original Message-----
> > From: Song Bao Hua (Barry Song)
> > Sent: Tuesday, December 22, 2020 3:03 PM
> > To: 'Vitaly Wool' <vitaly.wool@...sulko.com>
> > Cc: Shakeel Butt <shakeelb@...gle.com>; Minchan Kim <minchan@...nel.org>; Mike
> > Galbraith <efault@....de>; LKML <linux-kernel@...r.kernel.org>; linux-mm
> > <linux-mm@...ck.org>; Sebastian Andrzej Siewior <bigeasy@...utronix.de>;
> > NitinGupta <ngupta@...are.org>; Sergey Senozhatsky
> > <sergey.senozhatsky.work@...il.com>; Andrew Morton
> > <akpm@...ux-foundation.org>; tiantao (H) <tiantao6@...ilicon.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.

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
>
>