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

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Message-ID: <4BDE94C8.9000108@redhat.com>
Date:	Mon, 03 May 2010 12:18:00 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Jiri Slaby <jslaby@...e.cz>
CC:	Changli Gao <xiaosuo@...il.com>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	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 12:03 PM, Jiri Slaby wrote:
>
>    
>>> Because vmalloc is used to allocate virtually contiguous memory. v in
>>> vmalloc means virtually.
>>>
>>>
>>>        
>> A kmalloc()ed page is virtually contiguous, satisfying your requirement.
>>      
> But it won't work well for vmalloc_to_{page,pfn} and similar.

Modify vmalloc_to_{page,pfn} accordingly.

> Some code
> may expect vmalloc result to be in the vmalloc area and page-aligned
> (both in position and size).
>    

Both would be a bug IMO.  vmalloc() follows kmalloc() and malloc() which 
only guarantee natural alignment.

> Not that it won't be possible to inspect the callers, but in my eyes it
> would definitely be better to introduce kmalloc_or_vmalloc-alike where
> the caller explicitly doesn't care about the resulting position and size.
>    

You're forcing every user to make a choice about what's essentially a 
micro optimization.

If it's really needed to have a vmalloc-in-vmalloc-space, that should be 
a special function, while the ordinary vmalloc should make the optimization.

-- 
error compiling committee.c: too many arguments to function

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