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

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Date:   Sat, 19 Dec 2020 11:59:33 +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>
Subject: Re: [patch] zswap: fix zswap_frontswap_load() vs
 zsmalloc::map/unmap() might_sleep() splat

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