linux-kernel - Re: [PATCH 02/22] Do not sanity check order in the fast path

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Date:	Wed, 22 Apr 2009 09:13:11 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Linux Memory Management List <linux-mm@...ck.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lin Ming <ming.m.lin@...el.com>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 02/22] Do not sanity check order in the fast path

On Wed, 2009-04-22 at 14:53 +0100, Mel Gorman wrote:
> No user of the allocator API should be passing in an order >= MAX_ORDER
> but we check for it on each and every allocation. Delete this check and
> make it a VM_BUG_ON check further down the call path.

Should we get the check re-added to some of the upper-level functions,
then?  Perhaps __get_free_pages() or things like alloc_pages_exact()? 

I'm selfishly thinking of what I did in profile_init().  Can I slab
alloc it?  Nope.  Page allocator?  Nope.  Oh, well, try vmalloc():

        prof_buffer = kzalloc(buffer_bytes, GFP_KERNEL);
        if (prof_buffer)
                return 0;

        prof_buffer = alloc_pages_exact(buffer_bytes, GFP_KERNEL|__GFP_ZERO);
        if (prof_buffer)
                return 0;

        prof_buffer = vmalloc(buffer_bytes);
        if (prof_buffer)
                return 0;

        free_cpumask_var(prof_cpu_mask);
        return -ENOMEM;

Same thing in __kmalloc_section_memmap():

        page = alloc_pages(GFP_KERNEL|__GFP_NOWARN, get_order(memmap_size));
        if (page)
                goto got_map_page;

        ret = vmalloc(memmap_size);
        if (ret)
                goto got_map_ptr;

I depend on the allocator to tell me when I've fed it too high of an
order.  If we really need this, perhaps we should do an audit and then
add a WARN_ON() for a few releases to catch the stragglers.

-- Dave

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