linux-kernel - Re: [PATCH mm] mm: fix BUG with kvzalloc+GFP

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Message-ID: <YzHFK01dNy5dKJDO@linutronix.de>
Date:   Mon, 26 Sep 2022 17:28:43 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Florian Westphal <fw@...len.de>,
        Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        netdev@...r.kernel.org, netfilter-devel@...r.kernel.org,
        Martin Zaharinov <micron10@...il.com>
Subject: Re: [PATCH mm] mm: fix BUG with kvzalloc+GFP_ATOMIC

On 2022-09-26 17:03:48 [+0200], Vlastimil Babka wrote:
> > Doing the "p = kmalloc(sizeof(*p), GFP_ATOMIC);" from an atomic context
> > is also a problem nowadays. Such code should be fixed across the kernel
> > because of PREEMPT_RT support.

You should make sure that the context in question is atomic on
PREEMPT_RT before fixing it. My guess here is that it is average the
softirq (NAPI) callback which is fine.

> But the "atomic context" here is different, no? Calling kmalloc() from IRQ
> handlers AFAIK is ok as IRQ handlers are threaded on PREEMPT_RT. Calling it
> inside an local_irq_disable() would be a problem on the other hand. But then
> under e.g. spin_lock_irqsave() could be ok as those don't really disable
> irqs on RT.

correct.

Sebastian