[dpdk-dev] [RFC PATCH 2/2] lib/librte_eal: Remove unnecessary hugepage zero-filling

[Wang, Zhihong]

> -----Original Message-----
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Stephen Hemminger
> Sent: Thursday, November 19, 2015 3:09 AM
> To: Richardson, Bruce <bruce.richardson at intel.com>
> Cc: dev at dpdk.org
> Subject: Re: [dpdk-dev] [RFC PATCH 2/2] lib/librte_eal: Remove unnecessary
> hugepage zero-filling
> 
> On Wed, 18 Nov 2015 16:13:32 +0000
> "Richardson, Bruce" <bruce.richardson at intel.com> wrote:
> 
> >
> >
> > > -----Original Message-----
> > > From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Stephen
> > > Hemminger
> > > Sent: Wednesday, November 18, 2015 4:00 PM
> > > To: Xie, Huawei <huawei.xie at intel.com>
> > > Cc: dev at dpdk.org
> > > Subject: Re: [dpdk-dev] [RFC PATCH 2/2] lib/librte_eal: Remove
> > > unnecessary hugepage zero-filling
> > >
> > > On Wed, 18 Nov 2015 12:07:54 +0000
> > > "Xie, Huawei" <huawei.xie at intel.com> wrote:
> > >
> > > > >>> The kernel fills new allocated (huge) pages with zeros.
> > > > >>> DPDK just has to touch the pages to trigger the allocation.
> > > > I think we shouldn't reply on the assumption that kernel has
> > > > zeroed the memory. Kernel zeroes the memory mostly to avoid
> > > > information leakage.It could also achieve this by setting each bit to 1.
> > > > What we indeed need to check is later DPDK initialization code
> > > > doesn't assume the memory has been zeroed. Otherwise zero only
> > > > that part of the memory. Does this makes sense?
> > >
> > > If all new pages are zero, why does DPDK have to pre-touch the pages
> > > at all?
> >
> > The pages won't actually be mapped into the processes address space until
> accessed.
> >
> > /Bruce
> 
> Isn't that what mmap MAP_POPULATE flag (not currently used) will do.

Yes, the MAP_POPULATE does literally the same thing.
This flag is implemented since Linux 2.5.46 according to Linux man page, guess that's why DPDK fault the page tables manually in the first place. :)

I think we can use this flag since it makes the code clearer.

/Zhihong