linux-kernel - Re: [patch] zswap: fix zswap_frontswap_load() vs zsmalloc::map/unmap() might

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Date:   Sat, 19 Dec 2020 12:03:14 +0100
From:   Mike Galbraith <efault@....de>
To:     Vitaly Wool <vitaly.wool@...sulko.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Minchan Kim <minchan@...nel.org>,
        NitinGupta <ngupta@...are.org>
Subject: Re: [patch] zswap: fix zswap_frontswap_load() vs
 zsmalloc::map/unmap() might_sleep() splat

(CC zsmalloc maintainers)

On Sat, 2020-12-19 at 11:59 +0100, Mike Galbraith wrote:
> On Sat, 2020-12-19 at 11:46 +0100, Vitaly Wool wrote:
> > On Sat, 19 Dec 2020, 11:27 Mike Galbraith, <efault@....de> wrote:
> >
> > > The kernel that generated that splat was NOT an RT kernel, it was plain
> > > master.today with a PREEMPT config.
> >
> >
> > I see, thanks. I don't think it makes things better for zsmalloc
> > though. From what I can see, the offending code is this:
> >
> > >        /* From now on, migration cannot move the object */
> > >        pin_tag(handle);
> >
> > Bit spinlock is taken in pin_tag(). I find the comment above somewhat
> > misleading, why is it necessary to take a spinlock to prevent
> > migration? I would guess an atomic flag should normally be enough.
> >
> > zswap is not broken here, it is zsmalloc that needs to be fixed.
>
> Cool, those damn bit spinlocks going away would be a case of happiness
> for RT as well :)
>
> 	-Mike