linux-kernel - Re: [PATCH] zsmalloc: do not use bit_spin

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Date:   Sun, 20 Dec 2020 08:47:36 +0100
From:   Mike Galbraith <efault@....de>
To:     Vitaly Wool <vitaly.wool@...sulko.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>
Cc:     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] zsmalloc: do not use bit_spin_lock

On Sun, 2020-12-20 at 02:23 +0100, Mike Galbraith wrote:
> On Sun, 2020-12-20 at 02:22 +0200, Vitaly Wool wrote:
> > zsmalloc takes bit spinlock in its _map() callback and releases it
> > only in unmap() which is unsafe and leads to zswap complaining
> > about scheduling in atomic context.
> >
> > To fix that and to improve RT properties of zsmalloc, remove that
> > bit spinlock completely and use a bit flag instead.
>
> It also does get_cpu_var() in map(), put_cpu_var() in unmap().

That aside, the bit spinlock removal seems to hold up to beating in RT.
I stripped out the RT changes to replace the bit spinlocks, applied the
still needed atm might_sleep() fix, and ltp zram and zswap test are
running in a loop with no signs that it's a bad idea, so I hope that
makes it in (minus the preempt disabled spin which I whacked), as it
makes zsmalloc markedly more RT friendly.

RT changes go from:
 1 file changed, 79 insertions(+), 6 deletions(-)
to:
 1 file changed, 8 insertions(+), 3 deletions(-)

	-Mike