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

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Message-ID: <6946d6e798866276f0d635f219cdd4ad05156351.camel@gmx.de>
Date:   Sat, 19 Dec 2020 11:27:00 +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 11:20 +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.

	-Mike