linux-kernel - Re: [PATCH v3 06/12] ppc64/kexec_file: restrict memory usage of kdump kernel

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Date:   Fri, 17 Jul 2020 02:40:06 +0530
From:   Hari Bathini <hbathini@...ux.ibm.com>
To:     Thiago Jung Bauermann <bauerman@...ux.ibm.com>
Cc:     Pingfan Liu <piliu@...hat.com>, Nayna Jain <nayna@...ux.ibm.com>,
        Kexec-ml <kexec@...ts.infradead.org>,
        Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        lkml <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...abs.org>,
        Sourabh Jain <sourabhjain@...ux.ibm.com>,
        Petr Tesarik <ptesarik@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Young <dyoung@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>
Subject: Re: [PATCH v3 06/12] ppc64/kexec_file: restrict memory usage of kdump
 kernel



On 16/07/20 4:22 am, Thiago Jung Bauermann wrote:
> 
> Hari Bathini <hbathini@...ux.ibm.com> writes:
> 

<snip>

>> +/**
>> + * get_node_path - Get the full path of the given node.
>> + * @dn:            Node.
>> + * @path:          Updated with the full path of the node.
>> + *
>> + * Returns nothing.
>> + */
>> +static void get_node_path(struct device_node *dn, char *path)
>> +{
>> +	if (!dn)
>> +		return;
>> +
>> +	get_node_path(dn->parent, path);
> 
> Is it ok to do recursion in the kernel? In this case I believe it's not
> problematic since the maximum call depth will be the maximum depth of a
> device tree node which shouldn't be too much. Also, there are no local
> variables in this function. But I thought it was worth mentioning.

You are right. We are better off avoiding the recursion here. Will
change it to an iterative version instead.
 
>> +	 * each representing a memory range.
>> +	 */
>> +	ranges = (len >> 2) / (n_mem_addr_cells + n_mem_size_cells);
>> +
>> +	for (i = 0; i < ranges; i++) {
>> +		base = of_read_number(prop, n_mem_addr_cells);
>> +		prop += n_mem_addr_cells;
>> +		end = base + of_read_number(prop, n_mem_size_cells) - 1;

prop is not used after the above.

> You need to `prop += n_mem_size_cells` here.

But yeah, adding it would make it look complete in some sense..

Thanks
Hari