linux-kernel - Re: [PATCH 2/3] make new alloc_pages

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Date:	Mon, 11 Apr 2011 15:36:00 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Timur Tabi <timur@...escale.com>,
	Andi Kleen <andi@...stfloor.org>, Mel Gorman <mel@....ul.ie>,
	Michal Nazarewicz <mina86@...a86.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH 2/3] make new alloc_pages_exact()

On Mon, 2011-04-11 at 15:22 -0700, Andrew Morton wrote:
> > +/* 'struct page' version */
> > +struct page *__alloc_pages_exact(gfp_t gfp_mask, size_t size);
> > +void __free_pages_exact(struct page *page, size_t size);
> 
> The declarations use "size", but the definitions use "nr_pages". 
> "nr_pages" is way better.

I'll fix that.

> Should it really be size_t?  size_t's units are "bytes", usually.

Yeah, the nr_pages one should probably be an unsigned long.

> > -void *get_free_pages_exact(gfp_t gfp_mask, size_t size)
> > +struct page *__alloc_pages_exact(gfp_t gfp_mask, size_t nr_pages)
> 
> Most allocation functions are of the form foo(size, gfp_t), but this
> one has the args reversed.  Was there a reason for that?

I'm trying to make this a clone of alloc_pages(), which does:

	#define alloc_pages(gfp_mask, order)

It needs a note in the changelog on why I did it.

> >  {
> > -	unsigned int order = get_order(size);
> > -	unsigned long addr;
> > +	unsigned int order = get_order(nr_pages * PAGE_SIZE);
> > +	struct page *page;
> >  
> > -	addr = __get_free_pages(gfp_mask, order);
> > -	if (addr) {
> > -		unsigned long alloc_end = addr + (PAGE_SIZE << order);
> > -		unsigned long used = addr + PAGE_ALIGN(size);
> > +	page = alloc_pages(gfp_mask, order);
> > +	if (page) {
> > +		struct page *alloc_end = page + (1 << order);
> > +		struct page *used = page + nr_pages;
> >  
> > -		split_page(virt_to_page((void *)addr), order);
> > +		split_page(page, order);
> >  		while (used < alloc_end) {
> > -			free_page(used);
> > -			used += PAGE_SIZE;
> > +			__free_page(used);
> > +			used++;
> >  		}
> >  	}
> >  
> > -	return (void *)addr;
> > +	return page;
> > +}
> > +EXPORT_SYMBOL(__alloc_pages_exact);
> > +
> > +/**
> > + * __free_pages_exact - release memory allocated via __alloc_pages_exact()
> > + * @virt: the value returned by get_free_pages_exact.
> > + * @nr_pages: size in pages, same value as passed to __alloc_pages_exact().
> > + *
> > + * Release the memory allocated by a previous call to __alloc_pages_exact().
> > + */
> > +void __free_pages_exact(struct page *page, size_t nr_pages)
> > +{
> > +	struct page *end = page + nr_pages;
> > +
> > +	while (page < end) {
> 
> Hand-optimised.  Old school.  Doesn't trust the compiler :)

Hey, ask the dude who put free_pages_exact() in there! :)

> > +		__free_page(page);
> > +		page++;
> > +	}
> > +}
> > +EXPORT_SYMBOL(__free_pages_exact);
> 
> Really, this function duplicates release_pages().  release_pages() is
> big and fat and complex and is a crime against uniprocessor but it does
> make some effort to reduce the spinlocking frequency and in many
> situations, release_pages() will cause vastly less locked bus traffic
> than your __free_pages_exact().  And who knows, smart use of
> release_pages()'s "cold" hint may provide some benefits.

Seems like a decent enough thing to try.  I'll give it a shot and make
sure it's OK to use.

-- Dave

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