linux-kernel - Re: [PATCH v3] kexec: Make a pair of map and unmap reserved pages when kdump fails to start

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Message-ID: <20150710040527.GA3642@dhcp-128-21.nay.redhat.com>
Date:	Fri, 10 Jul 2015 12:05:27 +0800
From:	Minfei Huang <mnfhuang@...il.com>
To:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>, ebiederm@...ssion.com,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
	linux390@...ibm.com, linux-s390@...r.kernel.org,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] kexec: Make a pair of map and unmap reserved pages
 when kdump fails to start

On 07/09/15 at 05:54P, Michael Holzheu wrote:
> On Tue, 7 Jul 2015 17:18:40 -0400
> Vivek Goyal <vgoyal@...hat.com> wrote:
> 
> > On Thu, Jul 02, 2015 at 09:45:52AM +0800, Minfei Huang wrote:
> 
> [snip]
> 
> > I am thinking of moving kernel loading code in a separate function to
> > make things little simpler. Right now it is confusing.
> > 
> > Can you please test attached patch. I have only compile tested it. This
> > is primarily doing what you are doing but in a separate function. It
> > seems more readable now.
> 
> The patch looks good to me. What about the following patch on top
> to make things even more readable?
> ---
>  kernel/kexec.c |   50 +++++++++++++++++---------------------------------
>  1 file changed, 17 insertions(+), 33 deletions(-)
> 
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -1236,14 +1236,18 @@ int kexec_load_disabled;
>  
>  static DEFINE_MUTEX(kexec_mutex);
>  
> -static int __kexec_load(struct kimage **rimage, unsigned long entry,
> -			unsigned long nr_segments,
> +static int __kexec_load(unsigned long entry, unsigned long nr_segments,
>  			struct kexec_segment __user * segments,
>  			unsigned long flags)
>  {
> +	struct kimage *image, **dest_image;
>  	unsigned long i;
>  	int result;
> -	struct kimage *image;
> +
> +	dest_image = (flags & KEXEC_ON_CRASH) ? &kexec_crash_image : &kexec_image;
> +
> +	if (nr_segments == 0)
> +		return 0;

It is fine, if nr_segments is 0. So we should deal with this case like
original kexec code.

>  
>  	if (flags & KEXEC_ON_CRASH) {
>  		/*
> @@ -1251,7 +1255,6 @@ static int __kexec_load(struct kimage **
>  		 * crashes.  Free any current crash dump kernel before
>  		 * we corrupt it.
>  		 */
> -
>  		kimage_free(xchg(&kexec_crash_image, NULL));
>  	}
>  
> @@ -1267,30 +1270,29 @@ static int __kexec_load(struct kimage **
>  
>  	result = machine_kexec_prepare(image);
>  	if (result)
> -		goto out;
> +		goto fail;
>  
>  	for (i = 0; i < nr_segments; i++) {
>  		result = kimage_load_segment(image, &image->segment[i]);
>  		if (result)
> -			goto out;
> +			goto fail;
>  	}
> -
>  	kimage_terminate(image);
> -	*rimage = image;
> -out:
> +	/* Install the new kernel, and  uninstall the old */
> +	kimage_free(xchg(dest_image, image));
>  	if (flags & KEXEC_ON_CRASH)
>  		crash_unmap_reserved_pages();
> -
> -	/* Free image if there was an error */
> -	if (result)
> -		kimage_free(image);
> +	return 0;
> +fail:
> +	if (flags & KEXEC_ON_CRASH)
> +		crash_unmap_reserved_pages();
> +	kimage_free(image);

Kernel release image again, and will crash in here, since we do not
assign the image to NULL when we release the image above.

Thanks
Minfei

>  	return result;
>  }
>  
>  SYSCALL_DEFINE4(kexec_load, unsigned long, entry, unsigned long, nr_segments,
>  		struct kexec_segment __user *, segments, unsigned long, flags)
>  {
> -	struct kimage **dest_image, *image;
>  	int result;
>  
>  	/* We only trust the superuser with rebooting the system. */
> @@ -1315,9 +1317,6 @@ SYSCALL_DEFINE4(kexec_load, unsigned lon
>  	if (nr_segments > KEXEC_SEGMENT_MAX)
>  		return -EINVAL;
>  
> -	image = NULL;
> -	result = 0;
> -
>  	/* Because we write directly to the reserved memory
>  	 * region when loading crash kernels we need a mutex here to
>  	 * prevent multiple crash  kernels from attempting to load
> @@ -1329,24 +1328,9 @@ SYSCALL_DEFINE4(kexec_load, unsigned lon
>  	if (!mutex_trylock(&kexec_mutex))
>  		return -EBUSY;
>  
> -	dest_image = &kexec_image;
> -	if (flags & KEXEC_ON_CRASH)
> -		dest_image = &kexec_crash_image;
> -
>  	/* Load new kernel */
> -	if (nr_segments > 0) {
> -		result = __kexec_load(&image, entry, nr_segments, segments,
> -				      flags);
> -		if (result)
> -			goto out;
> -	}
> -
> -	/* Install the new kernel, and  Uninstall the old */
> -	image = xchg(dest_image, image);
> -
> -out:
> +	result = __kexec_load(entry, nr_segments, segments, flags);
>  	mutex_unlock(&kexec_mutex);
> -	kimage_free(image);
>  
>  	return result;
>  }
> 
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