linux-kernel - Re: [PATCH] mm/util.c: Make kvfree() safe for calling while holding spinlocks

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Date:   Thu, 23 Dec 2021 12:52:17 +0100
From:   Manfred Spraul <manfred@...orfullife.com>
To:     Vasily Averin <vvs@...tuozzo.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     cgel.zte@...il.com, shakeelb@...gle.com, rdunlap@...radead.org,
        dbueso@...e.de, unixbhaskar@...il.com, chi.minghao@....com.cn,
        arnd@...db.de, Zeal Robot <zealci@....com.cn>, linux-mm@...ck.org,
        1vier1@....de, stable@...r.kernel.org
Subject: Re: [PATCH] mm/util.c: Make kvfree() safe for calling while holding
 spinlocks

Hello Vasily,

On 12/23/21 08:21, Vasily Averin wrote:
>
> I would prefer to release memory ASAP if it's possible.
> What do you think about this change?
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -614,9 +614,12 @@ EXPORT_SYMBOL(kvmalloc_node);
>    */
>   void kvfree(const void *addr)
>   {
> -       if (is_vmalloc_addr(addr))
> -               vfree(addr);
> -       else
> +       if (is_vmalloc_addr(addr)) {
> +               if (in_atomic())
> +                       vfree_atomic();
> +               else
> +                       vfree(addr);
> +       } else
>                  kfree(addr);
>   }
>   EXPORT_SYMBOL(kvfree);
>
Unfortunately this cannot work:

> /*
> * Are we running in atomic context?  WARNING: this macro cannot
> * always detect atomic context; in particular, it cannot know about
> * held spinlocks in non-preemptible kernels.  Thus it should not be
> * used in the general case to determine whether sleeping is possible.
> * Do not use in_atomic() in driver code.
> */
> #define in_atomic()     (preempt_count() != 0)
>

--

     Manfred