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Subject: Re: [PATCH] fs: use kmalloc() to allocate fdmem if possible
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Jiri Slaby <jslaby@suse.cz>
Cc: Avi Kivity <avi@redhat.com>, Changli Gao <xiaosuo@gmail.com>,
       Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
       akpm@linux-foundation.org, paulmck@linux.vnet.ibm.com,
       adobriyan@gmail.com, mingo@elte.hu, peterz@infradead.org,
       linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
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Date: Mon, 03 May 2010 11:29:39 +0200
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Le lundi 03 mai 2010 à 11:19 +0200, Jiri Slaby a écrit :
> On 05/03/2010 11:13 AM, Eric Dumazet wrote:
> > /*
> >  * Warning: if size is not a power of two, kmalloc() might
> >  * waste memory because of its requirements.
> >  */
> > void *kvmalloc(size_t size)
> > {
> > 	void *ptr = kmalloc(size, GFP_KERNEL | __GFP_NOWARN);
> 
> This is kind of waste of high-order pages, isn't it? Wouldn't it be
> better to have a watermark where we skip kmalloc completely?

> 
> 

There are many drawbacks using kmalloc() instead of vmalloc().

(NUMA comes to mind, if you want a zone spreaded on all available nodes)

So this kvmalloc() is not meant to be a generic replacement of all
vmalloc() users, only selected ones.

In some spots, allocating 8192 bytes in a driver setup for example, it
should be fine. Using a high order page for this is perfect, as long as
we can have a fallback to vmalloc()

grep bnx2_alloc_mem /proc/vmallocinfo
0xf8690000-0xf8692000    8192 bnx2_alloc_mem+0x111/0x6d0 pages=1 vmalloc