linux-kernel - Re: [PATCH] staging: zcache: fix crash on cpu remove

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Date:	Thu, 06 Oct 2011 13:17:32 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
Cc:	gregkh@...e.de, cascardo@...oscopio.com,
	dan.magenheimer@...cle.com, rdunlap@...otime.net,
	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
	rcj@...ux.vnet.ibm.com, brking@...ux.vnet.ibm.com,
	ngupta@...are.org
Subject: Re: [PATCH] staging: zcache: fix crash on cpu remove

On Thu, 2011-10-06 at 14:28 -0500, Seth Jennings wrote:
> 
> +               if (kp->obj) {
> +                       kmem_cache_free(zcache_obj_cache, kp->obj);
> +                       kp->obj = NULL;
> +               }
> +               if (kp->page) {
> +                       free_page((unsigned long)kp->page);
> +                       kp->page = NULL;
> +               } 

Getting back from my preempt tangent... :)

I'm not sure how free_page() can oops.  If kp->page is NULL free_pages()
will rescue you:
        
        void free_pages(unsigned long addr, unsigned int order)
        {
                if (addr != 0) {
                        VM_BUG_ON(!virt_addr_valid((void *)addr));
                        __free_pages(virt_to_page((void *)addr), order);
                }
        }

I *thought* we did some of the same for kmem_cache_free(), but maybe
it's just kmalloc/kfree().  Anyway, it's all good.  I don't like relying
on the allocators to be nice when freeing NULL pointers anyway.

Acked-by: Dave Hansen <dave@...ux.vnet.ibm.com>

-- Dave


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