Subject: Re: [PATCH] ehea: Add hugepage detection

From: Thomas Klein <[email-masked]>
Date: Mon, 27 Oct 2008 10:33:47 +0100

Dave,

thanks for your valuable comments - see mine below. I'll send a reworked patch asap.

Thomas

On Friday 24 October 2008 08:55:37 pm you wrote:
> On Fri, 2008-10-24 at 13:07 +0200, Thomas Klein wrote:
> > 16GB hugepages may not be part of a memory region (firmware restriction). This patch
> > modifies the walk_memory_resource callback fn to filter hugepages and add only standard
> > memory to the busmap which is later on used for MR registration.
> 
> Does this support a case where a userspace app is reading network
> packets into a 16GB page backed area?  I think you need to elaborate on
> what kind of memory you need to have registered in these memory regions.
> It's hard to review what you've done here otherwise.
> 
> > --- linux-2.6.27/drivers/net/ehea/ehea_qmr.c	2008-10-24 09:29:19.000000000 +0200
> > +++ patched_kernel/drivers/net/ehea/ehea_qmr.c	2008-10-24 09:45:15.000000000 +0200
> > @@ -636,6 +636,9 @@ static int ehea_update_busmap(unsigned l
> >  {
> >  	unsigned long i, start_section, end_section;
> > 
> > +	if (!pgnum)
> > +		return 0;
> 
> This probably needs a comment.  It's not obvious what it is doing.

I decided to just rename the var to nr_pages as it is used in all other
busmap-related functions in our code. That makes the condition check quite
obvious.

> 
> >  	if (!ehea_bmap) {
> >  		ehea_bmap = kzalloc(sizeof(struct ehea_bmap), GFP_KERNEL);
> >  		if (!ehea_bmap)
> > @@ -692,10 +695,47 @@ int ehea_rem_sect_bmap(unsigned long pfn
> >  	return ret;
> >  }
> > 
> > -static int ehea_create_busmap_callback(unsigned long pfn,
> > -				       unsigned long nr_pages, void *arg)
> > +static int ehea_is_hugepage(unsigned long pfn)
> > +{
> > +	return ((((pfn << PAGE_SHIFT) & (EHEA_HUGEPAGE_SIZE - 1)) == 0)
> > +		&& (compound_order(pfn_to_page(pfn)) + PAGE_SHIFT
> > +		    == EHEA_HUGEPAGESHIFT) );
> > +}
> 
> Whoa.  That's dense.  Can you actually read that in less than 5 minutes?
> Seriously.

Thanks for this comment. I totally agree - and I'm happy to aerate it a bit.
I had been urged to make it dense during our internal review ;-)

> 
> I'm not sure what else you use EHEA_HUGEPAGE_SIZE for or if this gets
> duplicated, but this would look nicer if you just had a:
> 
> #define EHEA_HUGEPAGE_PFN_MASK ((EHEA_HUGEPAGE_SIZE - 1) >> PAGE_SHIFT)
> 
> 	if (pfn & EHEA_HUGEPAGE_PFN_MASK)
> 		return 0;
> 
> Or, with no new macro:
> 
> 	if ((pfn << PAGE_SHIFT) & (EHEA_HUGEPAGE_SIZE - 1) != 0)
> 		return 0;
> 
> 	page_order = compound_order(pfn_to_page(pfn));
> 	if (page_order + PAGE_SHIFT != EHEA_HUGEPAGESHIFT)
> 		return 0;
> 	return 1;
> }
> 
> Please break that up into something that is truly readable.  gcc will
> generate the exact same code.
> 
> > +static int ehea_create_busmap_callback(unsigned long initial_pfn,
> > +				       unsigned long total_nr_pages, void *arg)
> >  {
> > -	return ehea_update_busmap(pfn, nr_pages, EHEA_BUSMAP_ADD_SECT);
> > +	int ret;
> > +	unsigned long pfn, start_pfn, end_pfn, nr_pages;
> > +
> > +	if ((total_nr_pages * PAGE_SIZE) < EHEA_HUGEPAGE_SIZE)
> > +		return ehea_update_busmap(initial_pfn, total_nr_pages,
> > +					  EHEA_BUSMAP_ADD_SECT);
> > +
> > +	/* Given chunk is >= 16GB -> check for hugepages */
> > +	start_pfn = initial_pfn;
> > +	end_pfn = initial_pfn + total_nr_pages;
> > +	pfn = start_pfn;
> > +
> > +	while (pfn < end_pfn) {
> > +		if (ehea_is_hugepage(pfn)) {
> > +			/* Add mem found in front of the hugepage */
> > +			nr_pages = pfn - start_pfn;
> > +			ret = ehea_update_busmap(start_pfn, nr_pages,
> > +						 EHEA_BUSMAP_ADD_SECT);
> > +			if (ret)
> > +				return ret;
> > +
> > +			/* Skip the hugepage */
> > +			pfn += (EHEA_HUGEPAGE_SIZE / PAGE_SIZE);
> > +			start_pfn = pfn;
> > +		} else
> > +			pfn += (EHEA_SECTSIZE / PAGE_SIZE);
> > +	}
> > +
> > +	/* Add mem found behind the hugepage(s)  */
> > +	nr_pages = pfn - start_pfn;
> > +	return ehea_update_busmap(start_pfn, nr_pages, EHEA_BUSMAP_ADD_SECT);
> >  }
> > 
> >  int ehea_create_busmap(void)
> > diff -Nurp -X dontdiff linux-2.6.27/drivers/net/ehea/ehea_qmr.h patched_kernel/drivers/net/ehea/ehea_qmr.h
> > --- linux-2.6.27/drivers/net/ehea/ehea_qmr.h	2008-10-24 09:29:19.000000000 +0200
> > +++ patched_kernel/drivers/net/ehea/ehea_qmr.h	2008-10-24 09:45:15.000000000 +0200
> > @@ -40,6 +40,8 @@
> >  #define EHEA_PAGESIZE          (1UL << EHEA_PAGESHIFT)
> >  #define EHEA_SECTSIZE          (1UL << 24)
> >  #define EHEA_PAGES_PER_SECTION (EHEA_SECTSIZE >> EHEA_PAGESHIFT)
> > +#define EHEA_HUGEPAGESHIFT     34
> > +#define EHEA_HUGEPAGE_SIZE     (1UL << EHEA_HUGEPAGESHIFT)
> 
> I'm a bit worried that you're basically duplicating hugetlb.h here.  Why
> not just use the existing 16GB page macros?  While you're at it please
> expand these to give some more useful macros so you don't have to do
> arithmetic on them in the code as much.

I don't agree at this point. The 16GB hugepages we're dealing with here are
imho a different thing than the hugetlb stuff. Furthermore as far as I can see
the hugetlb macros vary depending on the kernel configuration while the ehea
driver requires them to be constant independently from the kernel config.
Please correct me if I missed something here.

> 
> #define EHEA_SECT_NR_PAGES (EHEA_SECTSIZE / PAGE_SIZE)
> 
> for instance.
> 
> -- Dave
> 
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