linux-kernel - Re: [PATCH] mm: Make kvfree safe to call

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Date:   Fri, 26 Jul 2019 14:10:18 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jeff Layton <jlayton@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Luis Henriques <lhenriques@...e.com>,
        Christoph Hellwig <hch@....de>,
        Carlos Maiolino <cmaiolino@...hat.com>
Subject: Re: [PATCH] mm: Make kvfree safe to call

On Fri, Jul 26, 2019 at 2:01 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
>
> Since vfree() can sleep, calling kvfree() from contexts where sleeping
> is not permitted (eg holding a spinlock) is a bit of a lottery whether
> it'll work.  Introduce kvfree_safe() for situations where we know we can
> sleep, but make kvfree() safe by default.
>
> Reported-by: Jeff Layton <jlayton@...nel.org>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: Luis Henriques <lhenriques@...e.com>
> Cc: Christoph Hellwig <hch@....de>
> Cc: Carlos Maiolino <cmaiolino@...hat.com>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>

So you say you are adding kvfree_safe() in the patch description, but
it looks like you are introducing kvfree_fast() below. Did something
change and the patch description wasn't updated, or is this just the
wrong description for this patch?

> ---
>  mm/util.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/mm/util.c b/mm/util.c
> index bab284d69c8c..992f0332dced 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -470,6 +470,28 @@ void *kvmalloc_node(size_t size, gfp_t flags, int node)
>  }
>  EXPORT_SYMBOL(kvmalloc_node);
>
> +/**
> + * kvfree_fast() - Free memory.
> + * @addr: Pointer to allocated memory.
> + *
> + * kvfree_fast frees memory allocated by any of vmalloc(), kmalloc() or
> + * kvmalloc().  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.  Must not
> + * hold a spinlock as it can sleep.
> + */
> +void kvfree_fast(const void *addr)
> +{
> +       might_sleep();
> +
> +       if (is_vmalloc_addr(addr))
> +               vfree(addr);
> +       else
> +               kfree(addr);
> +}
> +EXPORT_SYMBOL(kvfree_fast);
> +
>  /**
>   * kvfree() - Free memory.
>   * @addr: Pointer to allocated memory.
> @@ -478,12 +500,12 @@ 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.
>   */
>  void kvfree(const void *addr)
>  {
>         if (is_vmalloc_addr(addr))
> -               vfree(addr);
> +               vfree_atomic(addr);
>         else
>                 kfree(addr);
>  }
> --
> 2.20.1
>