Re: [patch] zswap: fix zswap_frontswap_load() vs zsmalloc::map/unmap() might_sleep() splat

[Mike Galbraith]

(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@xxxxxx> 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