Date: Wed, 24 Apr 2013 13:37:19 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: <hannes@cmpxchg.org>, <toshi.kani@hp.com>, <linuxram@us.ibm.com>,
        <rientjes@google.com>, <linux-kernel@vger.kernel.org>,
        <linux-mm@kvack.org>
Subject: Re: [Resend][Bug fix PATCH v5] Reusing a resource structure
 allocated by bootmem
Message-Id: <20130424133719.94c7d301df844c4bcc987a53@linux-foundation.org>
In-Reply-To: <51771E3D.6060203@jp.fujitsu.com>
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	<51771E3D.6060203@jp.fujitsu.com>
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On Wed, 24 Apr 2013 08:50:21 +0900 Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com> wrote:

> When hot removing memory presented at boot time, following messages are shown:
> 
> [  296.867031] ------------[ cut here ]------------
> [  296.922273] kernel BUG at mm/slub.c:3409!
>
> ...
>
> The reason why the messages are shown is to release a resource structure,
> allocated by bootmem, by kfree(). So when we release a resource structure,
> we should check whether it is allocated by bootmem or not.
> 
> But even if we know a resource structure is allocated by bootmem, we cannot
> release it since SLxB cannot treat it. So for reusing a resource structure,
> this patch remembers it by using bootmem_resource as follows:
> 
> When releasing a resource structure by free_resource(), free_resource() checks
> whether the resource structure is allocated by bootmem or not. If it is
> allocated by bootmem, free_resource() adds it to bootmem_resource. If it is
> not allocated by bootmem, free_resource() release it by kfree().
> 
> And when getting a new resource structure by get_resource(), get_resource()
> checks whether bootmem_resource has released resource structures or not. If
> there is a released resource structure, get_resource() returns it. If there is
> not a releaed resource structure, get_resource() returns new resource structure
> allocated by kzalloc().
> 
> ...
>

Looks good to me.

> --- a/kernel/resource.c
> +++ b/kernel/resource.c
> @@ -21,6 +21,7 @@
>  #include <linux/seq_file.h>
>  #include <linux/device.h>
>  #include <linux/pfn.h>
> +#include <linux/mm.h>
>  #include <asm/io.h>
>  
>  
> @@ -50,6 +51,14 @@ struct resource_constraint {
>  
>  static DEFINE_RWLOCK(resource_lock);
>  
> +/*
> + * For memory hotplug, there is no way to free resource entries allocated
> + * by boot mem after the system is up. So for reusing the resource entry
> + * we need to remember the resource.
> + */
> +static struct resource *bootmem_resource_free;
> +static DEFINE_SPINLOCK(bootmem_resource_lock);
> +
>  static void *r_next(struct seq_file *m, void *v, loff_t *pos)
>  {
>  	struct resource *p = v;
> @@ -151,6 +160,40 @@ __initcall(ioresources_init);
>  
>  #endif /* CONFIG_PROC_FS */
>  
> +static void free_resource(struct resource *res)
> +{
> +	if (!res)
> +		return;
> +
> +	if (!PageSlab(virt_to_head_page(res))) {

Did you consider using a bit in resource.flags?  There appear to be
four free ones left.  The VM trickery will work OK I guess, but isn't
very "nice".

> +		spin_lock(&bootmem_resource_lock);
> +		res->sibling = bootmem_resource_free;
> +		bootmem_resource_free = res;
> +		spin_unlock(&bootmem_resource_lock);
> +	} else {
> +		kfree(res);
> +	}
> +}
> +
> +static struct resource *get_resource(gfp_t flags)
> +{
> +	struct resource *res = NULL;
> +
> +	spin_lock(&bootmem_resource_lock);
> +	if (bootmem_resource_free) {
> +		res = bootmem_resource_free;
> +		bootmem_resource_free = res->sibling;
> +	}
> +	spin_unlock(&bootmem_resource_lock);
> +
> +	if (res)
> +		memset(res, 0, sizeof(struct resource));
> +	else
> +		res = kzalloc(sizeof(struct resource), flags);
> +
> +	return res;
> +}

I think I'll rename this to alloc_resource().  In Linux "get" often
(but not always) means "take a reference on".  So "get" pairs with
"put" and "alloc" pairs with "free".

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