Re: [RFC][PATCH 0/9] extend hugepage migration

[Michal Hocko <mhocko@suse.cz>]

On Fri 05-04-13 09:14:58, Simon Jeons wrote:
> Hi Michal,
> On 03/22/2013 04:15 PM, Michal Hocko wrote:
> >[getting off-list]
> >
> >On Fri 22-03-13 07:46:32, Simon Jeons wrote:
> >>Hi Michal,
> >>On 03/21/2013 08:56 PM, Michal Hocko wrote:
> >>>On Thu 21-03-13 07:49:48, Simon Jeons wrote:
> >>>[...]
> >>>>When I hacking arch/x86/mm/hugetlbpage.c like this,
> >>>>diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
> >>>>index ae1aa71..87f34ee 100644
> >>>>--- a/arch/x86/mm/hugetlbpage.c
> >>>>+++ b/arch/x86/mm/hugetlbpage.c
> >>>>@@ -354,14 +354,13 @@ hugetlb_get_unmapped_area(struct file *file,
> >>>>unsigned long addr,
> >>>>
> >>>>#endif /*HAVE_ARCH_HUGETLB_UNMAPPED_AREA*/
> >>>>
> >>>>-#ifdef CONFIG_X86_64
> >>>>static __init int setup_hugepagesz(char *opt)
> >>>>{
> >>>>unsigned long ps = memparse(opt, &opt);
> >>>>if (ps == PMD_SIZE) {
> >>>>hugetlb_add_hstate(PMD_SHIFT - PAGE_SHIFT);
> >>>>- } else if (ps == PUD_SIZE && cpu_has_gbpages) {
> >>>>- hugetlb_add_hstate(PUD_SHIFT - PAGE_SHIFT);
> >>>>+ } else if (ps == PUD_SIZE) {
> >>>>+ hugetlb_add_hstate(PMD_SHIFT - PAGE_SHIFT+4);
> >>>>} else {
> >>>>printk(KERN_ERR "hugepagesz: Unsupported page size %lu M\n",
> >>>>ps >> 20);
> >>>>
> >>>>I set boot=hugepagesz=1G hugepages=10, then I got 10 32MB huge pages.
> >>>>What's the difference between these pages which I hacking and normal
> >>>>huge pages?
> >>>How is this related to the patch set?
> >>>Please _stop_ distracting discussion to unrelated topics!
> >>>
> >>>Nothing personal but this is just wasting our time.
> >>Sorry kindly Michal, my bad.
> >>Btw, could you explain this question for me? very sorry waste your time.
> >Your CPU has to support GB pages. You have removed cpu_has_gbpages test
> >and added a hstate for order 13 pages which is a weird number on its
> >own (32MB) because there is no page table level to support them.
> 
> But after hacking, there is /sys/kernel/mm/hugepages/hugepages-*,
> and have equal number of 32MB huge pages which I set up in boot
> parameter.

because hugetlb_add_hstate creates hstate for those pages and
hugetlb_init_hstates allocates them later on.

> If there is no page table level to support them, how can
> them present?

Because hugetlb hstate handling code doesn't care about page tables and
the way how those pages are going to be mapped _at all_. Or put it in
another way. Nobody prevents you to allocate order-5 page for a single
pte but that would be a pure waste. Page fault code expects that pages
with a proper size are allocated.
-- 
Michal Hocko
SUSE Labs

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