linux-kernel - Re: max

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Message-Id: <20070322144210.73dfaf83.dada1@cosmosbay.com>
Date:	Thu, 22 Mar 2007 14:42:10 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Tomas M <tomas@...x.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: max_loop limit

On Thu, 22 Mar 2007 12:37:54 +0100
Tomas M <tomas@...x.org> wrote:

> The question is not "Why do we need more than 255 loops?".
> The question should be "Why do we need the hardcoded 255-limit in kernel 
> while there is no reason for it at all?"
> 
> My patch simply removes the hardcoded limitation.

Hello Tomas, welcome !

Well, its an attempt to remove a hardcoded limit, but as you said in the Changelog, it really depends on kmalloc() being able to allocate a large continous memory zone. Alas it might fail.
The golden rule is to avoid all allocations larger than PAGE_SIZE :)

On x86_64, sizeof(struct loop_device) is 368, so the 'new limit' would be 356 instead of 256...

You might want a more radical patch : 

Instead of using :

static struct loop_device *loop_dev;
loop_dev = kmalloc(max_loop * sizeof(struct loop_device));

Switch to :

static struct loop_device **loop_dev;
loop_dev = kmalloc(max_loop * sizeof(void *));
if (!loop_dev) rollback...
for (i = 0 ; i < max_loop ; i++) {
	loop_dev[i] = kmalloc(sizeof(struct loop_device));
	if (!loop_dev[i]) rollback...
}

This time, you would be limited to 16384 loop devices on x86_64, 32768 on i386 :)
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