linux-kernel - Re: [PATCH] slab: fix kmalloc_nolock() context check for PREEMPT

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Message-ID: <56598d76-02ca-490f-a100-0f223fb5717e@suse.cz>
Date: Mon, 5 Jan 2026 20:19:02 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Swaraj Gaikwad <swarajgaikwad1925@...il.com>, lgoncalv@...hat.com
Cc: akpm@...ux-foundation.org, ast@...nel.org, bigeasy@...utronix.de,
 cl@...two.org, clrkwllms@...nel.org, david.hunter.linux@...il.com,
 harry.yoo@...cle.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-rt-devel@...ts.linux.dev, rientjes@...gle.com,
 roman.gushchin@...ux.dev, rostedt@...dmis.org, skhan@...uxfoundation.org,
 syzbot+b1546ad4a95331b2101e@...kaller.appspotmail.com,
 Hao Li <hao.li@...ux.dev>
Subject: Re: [PATCH] slab: fix kmalloc_nolock() context check for PREEMPT_RT

On 12/19/25 14:51, Swaraj Gaikwad wrote:
> I agree that we can simplify this. I think !preemptible() would be
> the most descriptive choice here, as it directly expresses the
> constraint, we cannot take a sleeping lock when preemption is
> disabled.
> The updated check would be:
>     if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
> 
> I'll send a v2 with this change unless there are objections.

I think it's the best solution for now, so please do. We can change it back
later with sheaves, as Hao Li pointed out. It will limit PREEMPT_RT
kmalloc_nolock() for now. Steven explained to me it's the tracepoints
disabling preemption, but that will be also improved soon:

https://lore.kernel.org/all/20251219120607.7371034e@gandalf.local.home/


> Swaraj