On Wed, May 13, 2015 at 01:55:56PM -0700, Andrew Morton wrote:
> On Wed, 13 May 2015 01:44:22 +0000 Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> wrote:
>
> > > > order = huge_page_order(h);
> > > > VM_BUG_ON(!IS_ALIGNED(start_pfn, 1 << order));
> > > > + VM_BUG_ON(order == UINT_MAX);
> > > > for (pfn = start_pfn; pfn < end_pfn; pfn += 1 << order)
> > > > dissolve_free_huge_page(pfn_to_page(pfn));
> > >
> > > Do we need to calculate this each time? Can it be done in
> > > hugetlb_init_hstates(), save the result in a global?
> >
> > Yes, it should work. How about the following?
> > This adds 4bytes to .data due to a new global variable, but reduces 47 bytes
> > .text size of code reduces, so it's a win in total.
> >
> > text data bss dec hex filename
> > 28313 469 84236 113018 1b97a mm/hugetlb.o (above patch)
> > 28266 473 84236 112975 1b94f mm/hugetlb.o (below patch)
>
> Looks good. Please turn it into a real patch and send it over when
> convenient?
Yes, the patch is attached below.
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -40,6 +40,7 @@ int hugepages_treat_as_movable;
> > int hugetlb_max_hstate __read_mostly;
> > unsigned int default_hstate_idx;
> > struct hstate hstates[HUGE_MAX_HSTATE];
> > +unsigned int minimum_order __read_mostly;
>
> static.
>
> And a comment would be nice ;)
OK, fixed.
>
> >
> > ...
> >
> > @@ -1626,11 +1621,16 @@ static void __init hugetlb_init_hstates(void)
> > {
> > struct hstate *h;
> >
> > + minimum_order = UINT_MAX;
>
> Do this at compile time.
OK.
> > for_each_hstate(h) {
> > + if (minimum_order > huge_page_order(h))
> > + minimum_order = huge_page_order(h);
> > +
> > /* oversize hugepages were init'ed in early boot */
> > if (!hstate_is_gigantic(h))
> > hugetlb_hstate_alloc_pages(h);
> > }
> > + VM_BUG_ON(minimum_order == UINT_MAX);
>
> Is the system hopelessly screwed up when this happens, or will it still
> be able to boot up and do useful things?
>
> If the system is hopelessly broken then BUG_ON or, better, panic should
> be used here. But if there's still potential to do useful things then
> I guess VM_BUG_ON is appropriate.
When this happens, hugetlb subsystem is broken but the system can run as
usual, so this is not critical. So I think VM_BUG_ON is OK.
Thanks,
Naoya Horiguchi
---
From: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Subject: [PATCH] mm/hugetlb: introduce minimum hugepage order
Currently the initial value of order in dissolve_free_huge_page is 64 or 32,
which leads to the following warning in static checker:
mm/hugetlb.c:1203 dissolve_free_huge_pages()
warn: potential right shift more than type allows '9,18,64'
This is a potential risk of infinite loop, because 1 << order (== 0) is used
in for-loop like this:
for (pfn = start_pfn; pfn < end_pfn; pfn += 1 << order)
...
So this patch fixes it by using global minimum_order calculated at boot time.
text data bss dec hex filename
28313 469 84236 113018 1b97a mm/hugetlb.o
28256 473 84236 112965 1b945 mm/hugetlb.o (patched)
Fixes: c8721bbbdd36 ("mm: memory-hotplug: enable memory hotplug to handle hugepage")
Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
---
mm/hugetlb.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 271e4432734c..8c4c1f9f9a9a 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -40,6 +40,11 @@ int hugepages_treat_as_movable;
int hugetlb_max_hstate __read_mostly;
unsigned int default_hstate_idx;
struct hstate hstates[HUGE_MAX_HSTATE];
+/*
+ * Minimum page order among possible hugepage sizes, set to a proper value
+ * at boot time.
+ */
+static unsigned int minimum_order __read_mostly = UINT_MAX;
__initdata LIST_HEAD(huge_boot_pages);
@@ -1188,19 +1193,13 @@ static void dissolve_free_huge_page(struct page *page)
*/
void dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
{
- unsigned int order = 8 * sizeof(void *);
unsigned long pfn;
- struct hstate *h;
if (!hugepages_supported())
return;
- /* Set scan step to minimum hugepage size */
- for_each_hstate(h)
- if (order > huge_page_order(h))
- order = huge_page_order(h);
- VM_BUG_ON(!IS_ALIGNED(start_pfn, 1 << order));
- for (pfn = start_pfn; pfn < end_pfn; pfn += 1 << order)
+ VM_BUG_ON(!IS_ALIGNED(start_pfn, 1 << minimum_order));
+ for (pfn = start_pfn; pfn < end_pfn; pfn += 1 << minimum_order)
dissolve_free_huge_page(pfn_to_page(pfn));
}
@@ -1627,10 +1626,14 @@ static void __init hugetlb_init_hstates(void)
struct hstate *h;
for_each_hstate(h) {
+ if (minimum_order > huge_page_order(h))
+ minimum_order = huge_page_order(h);
+
/* oversize hugepages were init'ed in early boot */
if (!hstate_is_gigantic(h))
hugetlb_hstate_alloc_pages(h);
}
+ VM_BUG_ON(minimum_order == UINT_MAX);
}
static char * __init memfmt(char *buf, unsigned long n)
--
2.1.0
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