Re: [PATCH V2] mm/page_alloc: Ensure that HUGETLB_PAGE_ORDER is less than MAX_ORDER

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On 12.04.21 10:06, Anshuman Khandual wrote:
+ linuxppc-dev@xxxxxxxxxxxxxxxx
+ linux-ia64@xxxxxxxxxxxxxxx

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 <david@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
---
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.khandual@xxxxxxx/

  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






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