linux-kernel - Re: [PATCH] mm: vmalloc: Use the vmap_area_lock to protect ne_fit_preload

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Date:   Fri, 4 Oct 2019 18:30:42 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Daniel Wagner <dwagner@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: vmalloc: Use the vmap_area_lock to protect
 ne_fit_preload_node

On 2019-10-04 18:20:41 [+0200], Uladzislau Rezki wrote:
> On Fri, Oct 04, 2019 at 05:37:28PM +0200, Sebastian Andrzej Siewior wrote:
> > If you post something that is related to PREEMPT_RT please keep tglx and
> > me in Cc.
> > 
> > On 2019-10-03 11:09:06 [+0200], Daniel Wagner wrote:
> > > Replace preempt_enable() and preempt_disable() with the vmap_area_lock
> > > spin_lock instead. Calling spin_lock() with preempt disabled is
> > > illegal for -rt. Furthermore, enabling preemption inside the
> > 
> > Looking at it again, I have reasonable doubt that this
> > preempt_disable() is needed.
> > 
> The intention was to preload a current CPU with one extra object in
> non-atomic context, thus to use GFP_KERNEL permissive parameters. I.e.
> that allows us to avoid any allocation(if we stay on the same CPU)
> when we are in atomic context do splitting.

You could have been migrated to another CPU before the first
preempt_disable(). You could have been migrated to another CPU while
memory has been allocated.
I don't really see the point of that preempt_disable() besides keeping
debug code quiet.

> If we have migrate_disable/enable, then, i think preempt_enable/disable
> should be replaced by it and not the way how it has been proposed
> in the patch.

I don't think this patch is appropriate for upstream.

Sebastian