linux-kernel - Re: [PATCH] fs: use kmalloc() to allocate fdmem if possible

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Date:	Mon, 03 May 2010 09:06:18 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
CC:	xiaosuo@...il.com, jslaby@...e.cz, akpm@...ux-foundation.org,
	paulmck@...ux.vnet.ibm.com, adobriyan@...il.com, mingo@...e.hu,
	peterz@...radead.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: use kmalloc() to allocate fdmem if possible

On 05/03/2010 03:15 AM, Tetsuo Handa wrote:
> Avi Kivity wrote:
>    
>> On 05/02/2010 07:46 PM, Changli Gao wrote:
>>      
>>> use kmalloc() to allocate fdmem if possible.
>>>
>>> vmalloc() is used as a fallback solution for fdmem allocation. Two members are
>>> added into the hole of the structure fdtable to indicate if vmalloc() is used
>>> or not.
>>>
>>>
>>> diff --git a/fs/file.c b/fs/file.c
>>> index 34bb7f7..f71dd85 100644
>>> --- a/fs/file.c
>>> +++ b/fs/file.c
>>> @@ -39,28 +39,34 @@ int sysctl_nr_open_max = 1024 * 1024; /* raised later */
>>>     */
>>>    static DEFINE_PER_CPU(struct fdtable_defer, fdtable_defer_list);
>>>
>>> -static inline void * alloc_fdmem(unsigned int size)
>>> +static inline void *alloc_fdmem(unsigned int size, unsigned short *use_vmalloc)
>>>    {
>>> -	if (size<= PAGE_SIZE)
>>> -		return kmalloc(size, GFP_KERNEL);
>>> -	else
>>> -		return vmalloc(size);
>>> +	void *data;
>>> +
>>> +	data = kmalloc(size, GFP_KERNEL);
>>> +	if (data != NULL) {
>>> +		*use_vmalloc = 0;
>>> +		return data;
>>> +	}
>>> +	*use_vmalloc = 1;
>>> +
>>> +	return vmalloc(size);
>>>    }
>>>
>>>        
>> Perhaps vmalloc() should do this by itself?  vfree() can examine the
>> pointer and call kfree() if it isn't within the vmalloc address range.
>>      
> You can use is_vmalloc_addr().
>
> 	if (is_vmalloc_addr(p))
> 		vfree(p);
> 	else
> 		kfree(p);
>    


My point is, vmalloc() and vfree should do this, not their callers:

vmalloc(size):
     if (size <= PAGE_SIZE)
         return kmalloc(size, GFP_KERNEL);
     ...

vfree(p):
     if (!is_vmalloc_addr(p) {
         kfree(p);
         return;
     }
     ...

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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