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

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Date:   Sat, 19 Dec 2020 11:46:29 +0100
From:   Vitaly Wool <vitaly.wool@...sulko.com>
To:     Mike Galbraith <efault@....de>
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>
Subject: Re: [patch] zswap: fix zswap_frontswap_load() vs zsmalloc::map/unmap()
 might_sleep() splat

On Sat, 19 Dec 2020, 11:27 Mike Galbraith, <efault@....de> wrote:
>
> On Sat, 2020-12-19 at 11:20 +0100, Vitaly Wool wrote:
> > Hi Mike,
> >
> > On Sat, Dec 19, 2020 at 11:12 AM Mike Galbraith <efault@....de> wrote:
> > >
> > > (mailer partially munged formatting? resend)
> > >
> > > mm/zswap: fix zswap_frontswap_load() vs zsmalloc::map/unmap() might_sleep() splat
> > >
> > > zsmalloc map/unmap methods use preemption disabling bit spinlocks.  Take the
> > > mutex outside of pool map/unmap methods in zswap_frontswap_load() as is done
> > > in zswap_frontswap_store().
> >
> > oh wait... So is zsmalloc taking a spin lock in its map callback and
> > releasing it only in unmap? In this case, I would rather keep zswap as
> > is, mark zsmalloc as RT unsafe and have zsmalloc maintainer fix it.
>
> 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.

Best regards,
   Vitaly