linux-kernel - Re: [PATCH] mm/sparsemem: fix race in accessing memory

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Date:   Wed, 25 Oct 2023 14:35:41 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Charan Teja Kalla <quic_charante@...cinc.com>
Cc:     Pavan Kondeti <quic_pkondeti@...cinc.com>, <osalvador@...e.de>,
        <dan.j.williams@...el.com>, <david@...hat.com>, <vbabka@...e.cz>,
        <mgorman@...hsingularity.net>, <aneesh.kumar@...ux.ibm.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/sparsemem: fix race in accessing
 memory_section->usage

On Tue, 17 Oct 2023 19:40:15 +0530 Charan Teja Kalla <quic_charante@...cinc.com> wrote:

> >>  		 * was allocated during boot.
> >>  		 */
> >>  		if (!PageReserved(virt_to_page(ms->usage))) {
> >> +			synchronize_rcu();
> >>  			kfree(ms->usage);
> >>  			ms->usage = NULL;
> >>  		}
> > If we add NULL checks around ms->usage, this becomes
> > 
> > tmp = rcu_replace_pointer(ms->usage, NULL, hotplug_locked());
> > syncrhonize_rcu();
> > kfree(tmp);
> Per David input, I am working on using kfree_rcu().

How's it coming along?

Given that we're at 6.6-rc7 and given that this issue is causing
daily crashes in your device farm, I'm thinking that we use the current
version of your patch for 6.6 and for -stable.  We can look at the
kfree_rcu() optimization for later kernel releases?