On 12.04.21 10:06, Anshuman Khandual wrote:
+ linuxppc-...@lists.ozlabs.org
+ linux-i...@vger.kernel.org
On 4/12/21 9:18 AM, Anshuman Khandual wrote:
pageblock_order must always be less than MAX_ORDER, otherwise it might lead
to an warning during boot. A similar problem got fixed on arm64 platform
with the commit 79cc2ed5a716 ("arm64/mm: Drop THP conditionality from
FORCE_MAX_ZONEORDER"). Assert the above condition before HUGETLB_PAGE_ORDER
gets assigned as pageblock_order. This will help detect the problem earlier
on platforms where HUGETLB_PAGE_SIZE_VARIABLE is enabled.
Cc: David Hildenbrand <da...@redhat.com>
Cc: Andrew Morton <a...@linux-foundation.org>
Cc: linux...@kvack.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khand...@arm.com>
---
Changes in V2:
- Changed WARN_ON() to BUILD_BUG_ON() per David
Changes in V1:
https://patchwork.kernel.org/project/linux-mm/patch/1617947717-2424-1-git-send-email-anshuman.khand...@arm.com/
mm/page_alloc.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index cfc72873961d..19283bff4bec 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6875,10 +6875,17 @@ void __init set_pageblock_order(void)
if (pageblock_order)
return;
- if (HPAGE_SHIFT > PAGE_SHIFT)
+ if (HPAGE_SHIFT > PAGE_SHIFT) {
+ /*
+ * pageblock_order must always be less than
+ * MAX_ORDER. So does HUGETLB_PAGE_ORDER if
+ * that is being assigned here.
+ */
+ BUILD_BUG_ON(HUGETLB_PAGE_ORDER >= MAX_ORDER);
Unfortunately the build test fails on both the platforms (powerpc and ia64)
which subscribe HUGETLB_PAGE_SIZE_VARIABLE and where this check would make
sense. I some how overlooked the cross compile build failure that actually
detected this problem.
But wondering why this assert is not holding true ? and how these platforms
do not see the warning during boot (or do they ?) at mm/vmscan.c:1092 like
arm64 did.
static int __fragmentation_index(unsigned int order, struct contig_page_info
*info)
{
unsigned long requested = 1UL << order;
if (WARN_ON_ONCE(order >= MAX_ORDER))
return 0;
....
Can pageblock_order really exceed MAX_ORDER - 1 ?
Ehm, for now I was under the impression that such configurations
wouldn't exist.
And originally, HUGETLB_PAGE_SIZE_VARIABLE was introduced to handle
hugepage sizes that all *smaller* than MAX_ORDER - 1: See d9c234005227
("Do not depend on MAX_ORDER when grouping pages by mobility")
However, looking into init_cma_reserved_pageblock():
if (pageblock_order >= MAX_ORDER) {
i = pageblock_nr_pages;
...
}
But it's kind of weird, isn't it? Let's assume we have MAX_ORDER - 1
correspond to 4 MiB and pageblock_order correspond to 8 MiB.
Sure, we'd be grouping pages in 8 MiB chunks, however, we cannot even
allocate 8 MiB chunks via the buddy. So only alloc_contig_range() could
really grab them (IOW: gigantic pages).
Further, we have code like deferred_free_range(), where we end up
calling __free_pages_core()->...->__free_one_page() with
pageblock_order. Wouldn't we end up setting the buddy order to something
> MAX_ORDER -1 on that path?
Having pageblock_order > MAX_ORDER feels wrong and looks shaky.
--
Thanks,
David / dhildenb
