Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 26 Jan 2022 18:53:40 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Manfred Spraul <manfred@colorfullife.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Vasily Averin <vvs@virtuozzo.com>, cgel.zte@gmail.com,
        shakeelb@google.com, rdunlap@infradead.org, dbueso@suse.de,
        unixbhaskar@gmail.com, chi.minghao@zte.com.cn, arnd@arndb.de,
        Zeal Robot <zealci@zte.com.cn>, linux-mm@kvack.org,
        1vier1@web.de, stable@vger.kernel.org,
        Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH] mm/util.c: Make kvfree() safe for calling while holding
 spinlocks
Message-Id: <20220126185340.58f88e8e1b153b6650c83270@linux-foundation.org>
In-Reply-To: <20211222194828.15320-1-manfred@colorfullife.com>
References: <20211222194828.15320-1-manfred@colorfullife.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

On Wed, 22 Dec 2021 20:48:28 +0100 Manfred Spraul <manfred@colorfullife.com> wrote:

> One codepath in find_alloc_undo() calls kvfree() while holding a spinlock.
> Since vfree() can sleep this is a bug.
> 
> Previously, the code path used kfree(), and kfree() is safe to be called
> while holding a spinlock.
> 
> Minghao proposed to fix this by updating find_alloc_undo().
> 
> Alternate proposal to fix this: Instead of changing find_alloc_undo(),
> change kvfree() so that the same rules as for kfree() apply:
> Having different rules for kfree() and kvfree() just asks for bugs.
> 
> Disadvantage: Releasing vmalloc'ed memory will be delayed a bit.

I know we've been around this loop a bunch of times and deferring was
considered.   But I forget the conclusion.  IIRC, mhocko was involved?

> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -610,12 +610,12 @@ EXPORT_SYMBOL(kvmalloc_node);
>   * It is slightly more efficient to use kfree() or vfree() if you are certain
>   * that you know which one to use.
>   *
> - * Context: Either preemptible task context or not-NMI interrupt.
> + * Context: Any context except NMI interrupt.
>   */
>  void kvfree(const void *addr)
>  {
>  	if (is_vmalloc_addr(addr))
> -		vfree(addr);
> +		vfree_atomic(addr);
>  	else
>  		kfree(addr);
>  }