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Date: Mon, 27 Oct 2008 10:33:47 +0100
From: Thomas Klein <tklein@...ibm.com>
To: Dave Hansen <dave@...ux.vnet.ibm.com>
Cc: Christoph Raisch <raisch@...ibm.com>,
Hannes Hering <hering2@...ibm.com>,
"Jan-Bernd Themann" <themann@...ibm.com>,
"linux-kernel" <linux-kernel@...r.kernel.org>,
"linux-ppc" <linuxppc-dev@...abs.org>,
"netdev" <netdev@...r.kernel.org>, badari@...ibm.com,
haveblue@...ibm.com, tollefso@...ibm.com
Subject: Re: [PATCH] ehea: Add hugepage detection
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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