On 02/04/2015 14:41, Jay Rolette wrote: > On Thu, Apr 2, 2015 at 7:55 AM, Thomas Monjalon <thomas.monjalon at 6wind.com> > wrote: > >> 2015-04-02 19:30, jerry.lilijun at huawei.com: >>> From: Lilijun <jerry.lilijun at huawei.com> >>> >>> In the function map_all_hugepages(), hugepage memory is truly allocated >> by >>> memset(virtaddr, 0, hugepage_sz). Then it costs about 40s to finish the >>> dpdk memory initialization when 40000 2M hugepages are setup in host os. >> Yes it's something we should try to reduce. >> > I have a patch in my tree that does the same opto, but it is commented out > right now. In our case, 2/3's of the startup time for our entire app was > due to that particular call - memset(virtaddr, 0, hugepage_sz). Just > zeroing 1 byte per huge page reduces that by 30% in my tests. > > The only reason I have it commented out is that I didn't have time to make > sure there weren't side-effects for DPDK or my app. For normal shared > memory on Linux, pages are initialized to zero automatically once they are > touched, so the memset isn't required but I wasn't sure whether that > applied to huge pages. Also wasn't sure how hugetlbfs factored into the > equation. > > Hopefully someone can chime in on that. Would love to uncomment the opto :) > I think the opto/patch is good ;)
I had a look at the Linux kernel sources (mm/hugetlb.c)and at least since 2.6.32 (minimum Linux kernel version supported by DPDK) the kernel clears the hugepage (clear_huge_page) when it faults (hugetlb_no_page). Primary DPDK apps do clear_hugedir, clearing previously allocated hugepages, thus triggering hugepage faults (hugetlb_no_page) during map_all_hugepages. Note that even when we exit a primary DPDK app, hugepages remain allocated, reason why apps such as dump_cfg are able to retrieve config/memory information. Sergio >> In fact we can only write one byte to finish the allocation. >> >> Isn't it a security hole? >> > Not necessarily. If the kernel pre-zeros the huge pages via CoW like normal > pages, then definitely not. > > Even if the kernel doesn't pre-zero the pages, if DPDK takes care of > properly initializing memory structures on startup as they are carved out > of the huge pages, then it isn't a security hole. However, that approach is > susceptible to bit rot... You can audit the code and make sure everything > is kosher at first, but you have to worry about new code making assumptions > about how memory is initialized. > > >> This article speaks about "prezeroing optimizations" in Linux kernel: >> http://landley.net/writing/memory-faq.txt > > I read through that when I was trying to figure out what whether huge pages > were pre-zeroed or not. It doesn't talk about huge pages much beyond why > they are useful for reducing TLB swaps. > > Jay > >
